spim-manual - SPIM S20: A MIPS R2000 Simulator 1 \ 25 th...

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Unformatted text preview: SPIM S20: A MIPS R2000 Simulator 1 \ 25 th the performance at none of the cost" James R. Larus larus@cs.wisc.edu Computer Sciences Department University of Wisconsin{Madison 1210 West Dayton Street Madison, WI 53706, USA 608-262-9519 Copyright c 1990{1997 by James R. Larus (This document may be copied without royalties, so long as this copyright notice remains on it.) 1 SPIM SPIM S20 is a simulator that runs programs for the MIPS R2000/R3000 RISC computers.1 SPIM can read and immediately execute les containing assembly language. SPIM is a selfcontained system for running these programs and contains a debugger and interface to a few operating system services. The architecture of the MIPS computers is simple and regular, which makes it easy to learn and understand. The processor contains 32 general-purpose registers and a well-designed instruction set that make it a propitious target for generating code in a compiler. However, the obvious question is: why use a simulator when many people have workstations that contain a hardware, and hence signi cantly faster, implementation of this computer? One reason is that these workstations are not generally available. Another reason is that these machine will not persist for many years because of the rapid progress leading to new and faster computers. Unfortunately, the trend is to make computers faster by executing several instructions concurrently, which makes their architecture more di cult to understand and program. The MIPS architecture may be the epitome of a simple, clean RISC machine. In addition, simulators can provide a better environment for low-level programming than an actual machine because they can detect more errors and provide more features than an actual computer. For example, SPIM has a X-window interface that is better than most debuggers for the actual machines. I grateful to the many students at UW who used SPIM in their courses and happily found bugs in a professor's code. In particular, the students in CS536, Spring 1990, painfully found the last few bugs in an \already-debugged" simulator. I am grateful for their patience and persistence. Alan Yuen-wui Siow wrote the X-window interface. 1 For a description of the real machines, see Gerry Kane and Joe Heinrich, MIPS RISC Architecture, Prentice Hall, 1992. 1 Finally, simulators are an useful tool for studying computers and the programs that run on them. Because they are implemented in software, not silicon, they can be easily modi ed to add new instructions, build new systems such as multiprocessors, or simply to collect data. The MIPS architecture, like that of most RISC computers, is di cult to program directly because of its delayed branches, delayed loads, and restricted address modes. This di culty is tolerable since these computers were designed to be programmed in high-level languages and so present an interface designed for compilers, not programmers. A good part of the complexity results from delayed instructions. A delayed branch takes two cycles to execute. In the second cycle, the instruction immediately following the branch executes. This instruction can perform useful work that normally would have been done before the branch or it can be a nop (no operation). Similarly, delayed loads take two cycles so the instruction immediately following a load cannot use the value loaded from memory. MIPS wisely choose to hide this complexity by implementing a virtual machine with their assembler. This virtual computer appears to have non-delayed branches and loads and a richer instruction set than the actual hardware. The assembler reorganizes (rearranges) instructions to ll the delay slots. It also simulates the additional, pseudoinstructions by generating short sequences of actual instructions. By default, SPIM simulates the richer, virtual machine. It can also simulate the actual hardware. We will describe the virtual machine and only mention in passing features that do not belong to the actual hardware. In doing so, we are following the convention of MIPS assembly language programmers (and compilers), who routinely take advantage of the extended machine. Instructions marked with a dagger (y) are pseudoinstructions. 1.1 Simulation of a Virtual Machine 1.2 SPIM Interface SPIM provides a simple terminal and a X-window interface. Both provide equivalent functionality, but the X interface is generally easier to use and more informative. spim, the terminal version, and xspim, the X version, have the following command-line options: -bare Simulate a bare MIPS machine without pseudoinstructions or the additional addressing modes provided by the assembler. Implies -quiet. -asm Simulate the virtual MIPS machine provided by the assembler. This is the default. Accept pseudoinstructions in assembly code. Do not accept pseudoinstructions in assembly code. Do not load the standard trap handler. This trap handler has two functions that must be assumed by the user's program. First, it handles traps. When a trap occurs, SPIM jumps to location 0x80000080, which should contain code to service the exception. Second, 2 -pseudo -nopseudo -notrap this le contains startup code that invokes the routine main. Without the trap handler, execution begins at the instruction labeled start. -trap Load the standard trap handler. This is the default. Load the trap handler in the le. Print a message when an exception occurs. This is the default. Do not print a message at an exception. Disable the memory-mapped IO facility (see Section 5). -trap file -noquiet -quiet -nomapped io -mapped io Enable the memory-mapped IO facility (see Section 5). Programs that use SPIM syscalls (see Section 1.5) to read from the terminal should not also use memory-mapped IO. Load and execute the assembly code in the le. -s seg size Sets the initial size of memory segment seg to be size bytes. The memory segments are named: text, data, stack, ktext, and kdata. For example, the pair of arguments -sdata 2000000 starts the user data segment at 2,000,000 bytes. -lseg size Sets the limit on how large memory segment seg can grow to be size bytes. The memory segments that can grow are: data, stack, and kdata. -file 1.2.1 Terminal Interface The terminal interface (spim) provides the following commands: exit Exit the simulator. Read le of assembly language commands into SPIM's memory. If the le has already been read into SPIM, the system should be cleared (see reinitialize, below) or global symbols will be multiply de ned. Synonym for read. read "file" load "file" run <addr> Start running a program. If the optional address addr is provided, the program starts at that address. Otherwise, the program starts at the global symbol start, which is de ned by the default trap handler to call the routine at the global symbol main with the usual MIPS calling convention. 3 step <N> Step the program for N (default: 1) instructions. Print instructions as they execute. Continue program execution without stepping. Print register N . Print oating point register N . Print the contents of memory at address addr . Print the contents of the symbol table, i.e., the addresses of the global (but not local) symbols. Clear the memory and registers. Set a breakpoint at address addr . addr can be either a memory address or symbolic label. continue print $N print $fN print addr print sym reinitialize breakpoint addr delete addr list . Delete all breakpoints at address addr . List all breakpoints. Rest of line is an assembly instruction that is stored in memory. A newline reexecutes previous command. <nl> ? Print a help message. Most commands can be abbreviated to their unique pre x e.g., ex, re, l, dangerous commands, such as reinitialize, require a longer pre x. ru, s, p. More 1.2.2 X-Window Interface The X version of SPIM, xspim, looks di erent, but should operate in the same manner as spim. The X window has ve panes (see Figure 1). The top pane displays the contents of the registers. It is continually updated, except while a program is running. The next pane contains the buttons that control the simulator: quit Exit from the simulator. 4 xspim PC = 00000000 Status= 00000000 R0 R1 R2 R3 R4 R5 R6 R7 FP0 FP2 FP4 FP6 (r0) (at) (v0) (v1) (a0) (a1) (a2) (a3) = = = = = = = = = = = = 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 0.000000 0.000000 0.000000 0.000000 EPC HI R8 R9 R10 R11 R12 R13 R14 R15 FP8 FP10 FP12 FP14 = 00000000 = 00000000 (t0) (t1) (t2) (t3) (t4) (t5) (t6) (t7) = = = = = = = = = = = = Cause = 0000000 BadVaddr = 00000000 LO = 0000000 General Registers 00000000 R16 (s0) = 0000000 R24 (t8) = 00000000 00000000 R17 (s1) = 0000000 R25 (s9) = 00000000 00000000 R18 (s2) = 0000000 R26 (k0) = 00000000 00000000 R19 (s3) = 0000000 R27 (k1) = 00000000 00000000 R20 (s4) = 0000000 R28 (gp) = 00000000 00000000 R21 (s5) = 0000000 R29 (gp) = 00000000 00000000 R22 (s6) = 0000000 R30 (s8) = 00000000 00000000 R23 (s7) = 0000000 R31 (ra) = 00000000 Double Floating Point Registers 0.000000 FP16 = 0.00000 FP24 = 0.000000 0.000000 FP18 = 0.00000 FP26 = 0.000000 0.000000 FP20 = 0.00000 FP28 = 0.000000 0.000000 FP22 = 0.00000 FP30 = 0.000000 Single Floating Point Registers step terminal clear mode set value Register Display Control Buttons quit print load breakpt run help Text Segments User and Kernel Text Segments [0x00400000] [0x00400004] [0x00400008] [0x0040000c] [0x00400010] [0x00400014] [0x00400018] [0x0040001c] 0x8fa40000 0x27a50004 0x24a60004 0x00041090 0x00c23021 0x0c000000 0x3402000a 0x0000000c lw R4, 0(R29) addiu R5, R29, 4 addiu R6, R5, 4 sll R2, R4, 2 addu R6, R6, R2 jal 0x00000000 ori R0, R0, 10 syscall Data Segments [0x10000000]...[0x10010000] 0x00000000 [0x10010004] 0x74706563 0x206e6f69 0x636f2000 [0x10010010] 0x72727563 0x61206465 0x6920646e [0x10010020] 0x000a6465 0x495b2020 0x7265746e [0x10010030] 0x0000205d 0x20200000 0x616e555b [0x10010040] 0x61206465 0x65726464 0x69207373 [0x10010050] 0x642f7473 0x20617461 0x63746566 [0x10010060] 0x555b2020 0x696c616e 0x64656e67 [0x10010070] 0x73736572 0x206e6920 0x726f7473 SPIM Version 3.2 of January 14, 1990 Data and Stack Segments 0x726f6e67 0x74707572 0x6e67696c 0x6e69206e 0x00205d68 0x64646120 0x00205d65 SPIM Messages Figure 1: X-window interface to SPIM. 5 load run Read a source le into memory. Start the program running. step Single-step through a program. clear Reinitialize registers or memory. set value print Set the value in a register or memory location. Print the value in a register or memory location. breakpoint help Set or delete a breakpoint or list all breakpoints. Print a help message. terminal mode Raise or hide the console window. Set SPIM operating modes. The next two panes display the memory contents. The top one shows instructions from the user and kernel text segments.2 The rst few instructions in the text segment are startup code ( start) that loads argc and argv into registers and invokes the main routine. The lower of these two panes displays the data and stack segments. Both panes are updated as a program executes. The bottom pane is used to display messages from the simulator. It does not display output from an executing program. When a program reads or writes, its IO appears in a separate window, called the Console, which pops up when needed. Although SPIM faithfully simulates the MIPS computer, it is a simulator and certain things are not identical to the actual computer. The most obvious di erences are that instruction timing and the memory systems are not identical. SPIM does not simulate caches or memory latency, nor does it accurate re ect the delays for oating point operations or multiplies and divides. Another surprise (which occurs on the real machine as well) is that a pseudoinstruction expands into several machine instructions. When single-stepping or examining memory, the instructions that you see are slightly di erent from the source program. The correspondence between the two sets of instructions is fairly simple since SPIM does not reorganize the instructions to ll delay slots. These instructions are real|not pseudo|MIPS instructions. SPIM translates assembler pseudoinstructions to 1{3 MIPS instructions before storing the program in memory. Each source instruction appears as a comment on the rst instruction to which it is translated. 2 1.3 Surprising Features 6 Comments in assembler les begin with a sharp-sign (#). Everything from the sharp-sign to the end of the line is ignored. Identi ers are a sequence of alphanumeric characters, underbars ( ), and dots (.) that do not begin with a number. Opcodes for instructions are reserved words that are not valid identi ers. Labels are declared by putting them at the beginning of a line followed by a colon, for example: .data item: .word 1 .text .globl main main: lw $t0, item 1.4 Assembler Syntax # Must be global Strings are enclosed in double-quotes ("). Special characters in strings follow the C convention: newline tab quote \n \t \" SPIM supports a subset of the assembler directives provided by the MIPS assembler: .align n Align the next datum on a 2n byte boundary. For example, .align 2 aligns the next value on a word boundary. .align 0 turns o automatic alignment of .half, .word, .float, and .double directives until the next .data or .kdata directive. Store the string in memory, but do not null-terminate it. Store the string in memory and null-terminate it. Store the values in successive bytes of memory. n .ascii str .asciiz str .byte b1, ..., bn .data <addr> The following data items should be stored in the data segment. If the optional argument addr is present, the items are stored beginning at address addr . Store the n .double d1, ..., dn .extern sym size oating point double precision numbers in successive memory locations. Declare that the datum stored at sym is size bytes large and is a global symbol. This directive enables the assembler to store the datum in a portion of the data segment that is e ciently accessed via register $gp. Store the n .float f1, ..., fn .globl sym oating point single precision numbers in successive memory locations. Declare that symbol sym is global and can be referenced from other les. 7 print int print oat print double print string read int read oat read double read string sbrk exit Service System Call Code 1 2 3 4 5 6 7 8 9 10 $a0 = integer $f12 = oat $f12 = double $a0 = string Arguments Result $a0 $a0 = bu er, $a1 = length = amount address (in $v0) integer (in $v0) oat (in $f0) double (in $f0) Table 1: System services. .half h1, ..., hn n Store the 16-bit quantities in successive memory halfwords. The following data items should be stored in the kernel data segment. If the optional argument addr is present, the items are stored beginning at address addr . The next items are put in the kernel text segment. In SPIM, these items may only be instructions or words (see the .word directive below). If the optional argument addr is present, the items are stored beginning at address addr . n .kdata <addr> .ktext <addr> .space n Allocate SPIM). bytes of space in the current segment (which must be the data segment in .text <addr> The next items are put in the user text segment. In SPIM, these items may only be instructions or words (see the .word directive below). If the optional argument addr is present, the items are stored beginning at address addr . n Store the 32-bit quantities in successive memory words. SPIM does not distinguish various parts of the data segment (.data, .rdata, and .sdata). SPIM provides a small set of operating-system-like services through the system call (syscall) instruction. To request a service, a program loads the system call code (see Table 1) into register $v0 and the arguments into registers $a0 $a3 (or $f12 for oating point values). System calls that return values put their result in register $v0 (or $f0 for oating point results). For example, to print \the answer = 5", use the commands: ::: .word w1, ..., wn 1.5 System Calls str: .data .asciiz "the answer = " 8 Memory CPU Registers $0 . . . $31 Arithmetic Unit Multiply Divide Lo Hi FPU (Coprocessor 1) Registers $0 . . . $31 Arithmetic Unit Coprocessor 0 (Traps and Memory) BadVAddr Status Cause EPC Figure 2: MIPS R2000 CPU and FPU .text li $v0, 4 la $a0, str syscall li $v0, 1 li $a0, 5 syscall print int # system call code for print_str # address of string to print # print the string # system call code for print_int # integer to print # print it is passed an integer and prints it on the console. print float prints a single oating point number. print double prints a double precision number. print string is passed a pointer to a null-terminated string, which it writes to the console. read int, read float, and read double read an entire line of input up to and including the newline. Characters following the number are ignored. read string has the same semantics as the Unix library routine fgets. It reads up to ; 1 characters into a bu er and terminates the string with a null byte. If there are fewer characters on the current line, it reads through the newline and again null-terminates the string. Warning: programs that use these syscalls to read from the terminal should not use memory-mapped IO (see Section 5). sbrk returns a pointer to a block of memory containing additional bytes. exit stops a program from running. n n 9 Register Name Number zero at v0 v1 a0 a1 a2 a3 t0 t1 t2 t3 t4 t5 t6 t7 s0 s1 s2 s3 s4 s5 s6 s7 t8 t9 k0 k1 gp sp fp ra 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 Constant 0 Reserved for assembler Expression evaluation and results of a function Argument 1 Argument 2 Argument 3 Argument 4 Temporary (not preserved across call) Temporary (not preserved across call) Temporary (not preserved across call) Temporary (not preserved across call) Temporary (not preserved across call) Temporary (not preserved across call) Temporary (not preserved across call) Temporary (not preserved across call) Saved temporary (preserved across call) Saved temporary (preserved across call) Saved temporary (preserved across call) Saved temporary (preserved across call) Saved temporary (preserved across call) Saved temporary (preserved across call) Saved temporary (preserved across call) Saved temporary (preserved across call) Temporary (not preserved across call) Temporary (not preserved across call) Reserved for OS kernel Reserved for OS kernel Pointer to global area Stack pointer Frame pointer Return address (used by function call) Usage Table 2: MIPS registers and the convention governing their use. 2 Description of the MIPS R2000 A MIPS processor consists of an integer processing unit (the CPU) and a collection of coprocessors that perform ancillary tasks or operate on other types of data such as oating point numbers (see Figure 2). SPIM simulates two coprocessors. Coprocessor 0 handles traps, exceptions, and the virtual memory system. SPIM simulates most of the rst two and entirely omits details of the memory system. Coprocessor 1 is the oating point unit. SPIM simulates most aspects of this unit. The MIPS (and SPIM) central processing unit contains 32 general purpose registers that are numbered 0{31. Register is designated by $n. Register $0 always contains the hardwired value 0. MIPS has established a set of conventions as to how registers should be used. These n 2.1 CPU Registers 10 15 10 5 4 3 2 1 0 In E n te ab rru le pt K U ern se e r l/ Figure 3: The Status register. suggestions are guidelines, which are not enforced by the hardware. However a program that violates them will not work properly with other software. Table 2 lists the registers and describes their intended use. Registers $at (1), $k0 (26), and $k1 (27) are reserved for use by the assembler and operating system. Registers $a0{$a3 (4{7) are used to pass the rst four arguments to routines (remaining arguments are passed on the stack). Registers $v0 and $v1 (2, 3) are used to return values from functions. Registers $t0{$t9 (8{15, 24, 25) are caller-saved registers used for temporary quantities that do not need to be preserved across calls. Registers $s0{$s7 (16{23) are calleesaved registers that hold long-lived values that should be preserved across calls. Register $sp (29) is the stack pointer, which points to the last location in use on the stack.3 Register $fp (30) is the frame pointer.4 Register $ra (31) is written with the return address for a call by the jal instruction. Register $gp (28) is a global pointer that points into the middle of a 64K block of memory in the heap that holds constants and global variables. The objects in this heap can be quickly accessed with a single load or store instruction. In addition, coprocessor 0 contains registers that are useful to handle exceptions. SPIM does not implement all of these registers, since they are not of much use in a simulator or are part of the memory system, which is not implemented. However, it does provide the following: Register Name Number BadVAddr Status Cause EPC 8 12 13 14 Memory address at which address exception occurred Interrupt mask and enable bits Exception type and pending interrupt bits Address of instruction that caused exception Usage These registers are part of coprocessor 0's register set and are accessed by the lwc0, mfc0, mtc0, and swc0 instructions. Figure 3 describes the bits in the Status register that are implemented by SPIM. The interrupt mask contains a bit for each of the ve interrupt levels. If a bit is one, interrupts at that level are allowed. If the bit is zero, interrupts at that level are disabled. The low six bits of the Status register implement a three-level stack for the kernel/user and interrupt enable bits. The kernel/user bit is 0 if the program was running in the kernel when the interrupt 3 In earlier version of SPIM, $sp was documented as pointing at the rst free word on the stack (not the last word of the stack frame). Recent MIPS documents have made it clear that this was an error. Both conventions work equally well, but we choose to follow the real system. 4 The MIPS compiler does not use a frame pointer, so this register is used as callee-saved register $s8. 11 In E n te ab rru le pt K U ern se e r l/ K U ern se e r l/ In E n te ab rru le pt Interrupt Mask Old Previous Current 15 10 5 2 Pending Interrupts Exception Code Figure 4: The Cause register. occurred and 1 if it was in user mode. If the interrupt enable bit is 1, interrupts are allowed. If it is 0, they are disabled. At an interrupt, these six bits are shifted left by two bits, so the current bits become the previous bits and the previous bits become the old bits. The current bits are both set to 0 (i.e., kernel mode with interrupts disabled). Figure 4 describes the bits in the Cause registers. The ve pending interrupt bits correspond to the ve interrupt levels. A bit becomes 1 when an interrupt at its level has occurred but has not been serviced. The exception code register contains a code from the following table describing the cause of an exception. Number Name 0 4 5 6 7 8 9 10 12 INT ADDRL ADDRS IBUS DBUS SYSCALL BKPT RI OVF Description External interrupt Address error exception (load or instruction fetch) Address error exception (store) Bus error on instruction fetch Bus error on data load or store Syscall exception Breakpoint exception Reserved instruction exception Arithmetic over ow exception 2.2 Byte Order Processors can number the bytes within a word to make the byte with the lowest number either the leftmost or rightmost one. The convention used by a machine is its byte order . MIPS processors can operate with either big-endian byte order: 0 1 2 3 or little-endian byte order: 3 2 1 0 SPIM operates with both byte orders. SPIM's byte order is determined by the byte order of the underlying hardware running the simulator. On a DECstation 3100, SPIM is little-endian, while on a HP Bobcat, Sun 4 or PC/RT, SPIM is big-endian. Byte # Byte # 2.3 Addressing Modes MIPS is a load/store architecture, which means that only load and store instructions access memory. Computation instructions operate only on values in registers. The bare machine 12 provides only one memory addressing mode: c(rx), which uses the sum of the immediate (integer) c and the contents of register rx as the address. The virtual machine provides the following addressing modes for load and store instructions: (register) imm imm (register) symbol symbol imm symbol imm (register) Format contents of register immediate immediate + contents of register address of symbol address of symbol + or ; immediate address of symbol + or ; (immediate + contents of register) Address Computation Most load and store instructions operate only on aligned data. A quantity is aligned if its memory address is a multiple of its size in bytes. Therefore, a halfword object must be stored at even addresses and a full word object must be stored at addresses that are a multiple of 4. However, MIPS provides some instructions for manipulating unaligned data. In all instructions below, Src2 can either be a register or an immediate value (a 16 bit integer). The immediate forms of the instructions are only included for reference. The assembler will translate the more general form of an instruction (e.g., add) into the immediate form (e.g., addi) if the second argument is constant. abs Rdest, Rsrc 2.4 Arithmetic and Logical Instructions Put the absolute value of the integer from register Rsrc in register Rdest. add Rdest, Rsrc1, Src2 addi Rdest, Rsrc1, Imm addu Rdest, Rsrc1, Src2 addiu Rdest, Rsrc1, Imm Absolute Value y ow) ow) ow) ow) Addition (with over Addition Immediate (with over Addition (without over Addition Immediate (without over Put the sum of the integers from register Rsrc1 and Src2 (or Imm) into register Rdest. AND AND Immediate Put the logical AND of the integers from register Rsrc1 and Src2 (or Imm) into register Rdest. and Rdest, Rsrc1, Src2 andi Rdest, Rsrc1, Imm Divide (signed) Divide (unsigned) Divide the contents of the two registers. divu treats is operands as unsigned values. Leave the quotient in register lo and the remainder in register hi. Note that if an operand is negative, the remainder is unspeci ed by the MIPS architecture and depends on the conventions of the machine on which SPIM is run. div Rsrc1, Rsrc2 divu Rsrc1, Rsrc2 Divide (signed, with over ow) y Divide (unsigned, without over ow) y Put the quotient of the integers from register Rsrc1 and Src2 into register Rdest. divu treats is operands as unsigned values. div Rdest, Rsrc1, Src2 divu Rdest, Rsrc1, Src2 mul Rdest, Rsrc1, Src2 mulo Rdest, Rsrc1, Src2 Multiply (without over ow) y Multiply (with over ow) y 13 Unsigned Multiply (with over ow) y Put the product of the integers from register Rsrc1 and Src2 into register Rdest. mulou Rdest, Rsrc1, Src2 mult Rsrc1, Rsrc2 multu Rsrc1, Rsrc2 Multiply Unsigned Multiply Multiply the contents of the two registers. Leave the low-order word of the product in register lo and the high-word in register hi. Negate Value (with over ow) y Negate Value (without over ow) y Put the negative of the integer from register Rsrc into register Rdest. neg Rdest, Rsrc negu Rdest, Rsrc Put the logical NOR of the integers from register Rsrc1 and Src2 into register Rdest. not Rdest, Rsrc nor Rdest, Rsrc1, Src2 NOR NOT y Put the bitwise logical negation of the integer from register Rsrc into register Rdest. or Rdest, Rsrc1, Src2 ori Rdest, Rsrc1, Imm OR OR Immediate Put the logical OR of the integers from register Rsrc1 and Src2 (or Imm) into register Rdest. Remainder y Unsigned Remainder y Put the remainder from dividing the integer in register Rsrc1 by the integer in Src2 into register Rdest. Note that if an operand is negative, the remainder is unspeci ed by the MIPS architecture and depends on the conventions of the machine on which SPIM is run. rem Rdest, Rsrc1, Src2 remu Rdest, Rsrc1, Src2 rol Rdest, Rsrc1, Src2 ror Rdest, Rsrc1, Src2 Rotate Left y Rotate Right y Rotate the contents of register Rsrc1 left (right) by the distance indicated by Src2 and put the result in register Rdest. sll Rdest, Rsrc1, Src2 sllv Rdest, Rsrc1, Rsrc2 sra Rdest, Rsrc1, Src2 srav Rdest, Rsrc1, Rsrc2 srl Rdest, Rsrc1, Src2 srlv Rdest, Rsrc1, Rsrc2 Shift Left Logical Shift Left Logical Variable Shift Right Arithmetic Shift Right Arithmetic Variable Shift Right Logical Shift Right Logical Variable Shift the contents of register Rsrc1 left (right) by the distance indicated by Src2 (Rsrc2) and put the result in register Rdest. Subtract (with over ow) Subtract (without over ow) Put the di erence of the integers from register Rsrc1 and Src2 into register Rdest. sub Rdest, Rsrc1, Src2 subu Rdest, Rsrc1, Src2 XOR XOR Immediate Put the logical XOR of the integers from register Rsrc1 and Src2 (or Imm) into register Rdest. xor Rdest, Rsrc1, Src2 xori Rdest, Rsrc1, Imm 14 2.5 Constant-Manipulating Instructions Move the immediate imm into register Rdest. lui Rdest, imm li Rdest, imm Load Immediate y Load Upper Immediate Load the lower halfword of the immediate imm into the upper halfword of register Rdest. The lower bits of the register are set to 0. 2.6 Comparison Instructions In all instructions below, Src2 can either be a register or an immediate value (a 16 bit integer). seq Rdest, Rsrc1, Src2 Set register Rdest to 1 if register Rsrc1 sge Rdest, Rsrc1, Src2 sgeu Rdest, Rsrc1, Src2 Set register Rdest to 1 if register Rsrc1 sgt Rdest, Rsrc1, Src2 sgtu Rdest, Rsrc1, Src2 Set register Rdest to 1 if register Rsrc1 sle Rdest, Rsrc1, Src2 sleu Rdest, Rsrc1, Src2 Set register Rdest to 1 if register Rsrc1 slt Rdest, Rsrc1, Src2 slti Rdest, Rsrc1, Imm sltu Rdest, Rsrc1, Src2 sltiu Rdest, Rsrc1, Imm Set register Rdest to 1 if register Rsrc1 sne Rdest, Rsrc1, Src2 Set register Rdest to 1 if register Rsrc1 equals Src2 and to be 0 otherwise. Set Equal y Set Greater Than Equal y Set Greater Than Equal Unsigned y is greater than or equal to Src2 and to 0 otherwise. Set Greater Than y Set Greater Than Unsigned y is greater than Src2 and to 0 otherwise. Set Less Than Equal y Set Less Than Equal Unsigned y is less than or equal to Src2 and to 0 otherwise. Set Less Than Set Less Than Immediate Set Less Than Unsigned Set Less Than Unsigned Immediate is less than Src2 (or Imm) and to 0 otherwise. Set Not Equal y is not equal to Src2 and to 0 otherwise. 2.7 Branch and Jump Instructions In all instructions below, Src2 can either be a register or an immediate value (integer). Branch instructions use a signed 16-bit o set eld hence they can jump 215 ; 1 instructions (not bytes) forward or 215 instructions backwards. The jump instruction contains a 26 bit address eld. Unconditionally branch to the instruction at the label. bczt label bczf label b label Branch instruction y Branch Coprocessor z True Branch Coprocessor z False Conditionally branch to the instruction at the label if coprocessor z 's condition ag is true (false). 15 beq Rsrc1, Src2, label Branch on Equal Conditionally branch to the instruction at the label if the contents of register Rsrc1 equals Src2. Branch on Equal Zero y Conditionally branch to the instruction at the label if the contents of Rsrc equals 0. beqz Rsrc, label Branch on Greater Than Equal y Branch on GTE Unsigned y Conditionally branch to the instruction at the label if the contents of register Rsrc1 are greater than or equal to Src2. bge Rsrc1, Src2, label bgeu Rsrc1, Src2, label Branch on Greater Than Equal Zero Conditionally branch to the instruction at the label if the contents of Rsrc are greater than or equal to 0. bgez Rsrc, label bgezal Rsrc, label Branch on Greater Than Equal Zero And Link Conditionally branch to the instruction at the label if the contents of Rsrc are greater than or equal to 0. Save the address of the next instruction in register 31. bgt Rsrc1, Src2, label bgtu Rsrc1, Src2, label Branch on Greater Than y Branch on Greater Than Unsigned y Conditionally branch to the instruction at the label if the contents of register Rsrc1 are greater than Src2. bgtz Rsrc, label Branch on Greater Than Zero Conditionally branch to the instruction at the label if the contents of Rsrc are greater than 0. Branch on Less Than Equal y Branch on LTE Unsigned y Conditionally branch to the instruction at the label if the contents of register Rsrc1 are less than or equal to Src2. ble Rsrc1, Src2, label bleu Rsrc1, Src2, label Branch on Less Than Equal Zero Conditionally branch to the instruction at the label if the contents of Rsrc are less than or equal to 0. blez Rsrc, label Branch on Greater Than Equal Zero And Link Branch on Less Than And Link Conditionally branch to the instruction at the label if the contents of Rsrc are greater or equal to 0 or less than 0, respectively. Save the address of the next instruction in register 31. bgezal Rsrc, label bltzal Rsrc, label Branch on Less Than y Branch on Less Than Unsigned y Conditionally branch to the instruction at the label if the contents of register Rsrc1 are less than Src2. blt Rsrc1, Src2, label bltu Rsrc1, Src2, label Branch on Less Than Zero Conditionally branch to the instruction at the label if the contents of Rsrc are less than 0. bltz Rsrc, label 16 Branch on Not Equal Conditionally branch to the instruction at the label if the contents of register Rsrc1 are not equal to Src2. bne Rsrc1, Src2, label Branch on Not Equal Zero y Conditionally branch to the instruction at the label if the contents of Rsrc are not equal to 0. bnez Rsrc, label Unconditionally jump to the instruction at the label. jal label jalr Rsrc j label Jump Jump and Link Jump and Link Register Unconditionally jump to the instruction at the label or whose address is in register Rsrc. Save the address of the next instruction in register 31. jr Rsrc Unconditionally jump to the instruction whose address is in register Rsrc. Jump Register 2.8 Load Instructions la Rdest, address Load computed address , not the contents of the location, into register Rdest. Load Address y lb Rdest, address lbu Rdest, address Load Byte Load Unsigned Byte Load the byte at address into register Rdest. The byte is sign-extended by the lb, but not the lbu, instruction. ld Rdest, address + 1. Load the 64-bit quantity at address into registers Rdest and Rdest lh Rdest, address lhu Rdest, address Load Double-Word y Load Halfword Load Unsigned Halfword Load the 16-bit quantity (halfword) at address into register Rdest. The halfword is sign-extended by the lh, but not the lhu, instruction lw Rdest, address Load the 32-bit quantity (word) at address into register Rdest. lwcz Rdest, address Load Word Load Word Coprocessor Load the word at address into register Rdest of coprocessor (0{3). z Load Word Left Load Word Right Load the left (right) bytes from the word at the possibly-unaligned address into register Rdest. lwl Rdest, address lwr Rdest, address ulh Rdest, address ulhu Rdest, address Unaligned Load Halfword y Unaligned Load Halfword Unsigned y 17 Load the 16-bit quantity (halfword) at the possibly-unaligned address into register Rdest. The halfword is sign-extended by the ulh, but not the ulhu, instruction Unaligned Load Word y Load the 32-bit quantity (word) at the possibly-unaligned address into register Rdest. ulw Rdest, address 2.9 Store Instructions sb Rsrc, address Store the low byte from register Rsrc at address . sd Rsrc, address + 1 Store Byte Store the 64-bit quantity in registers Rsrc and Rsrc sh Rsrc, address at address . Store Double-Word y Store Halfword Store Word Store Word Coprocessor Store the low halfword from register Rsrc at address . sw Rsrc, address Store the word from register Rsrc at address . z swcz Rsrc, address Store the word from register Rsrc of coprocessor at address . swl Rsrc, address swr Rsrc, address Store Word Left Store Word Right Store the left (right) bytes from register Rsrc at the possibly-unaligned address . Unaligned Store Halfword y Store the low halfword from register Rsrc at the possibly-unaligned address . ush Rsrc, address Store the word from register Rsrc at the possibly-unaligned address . usw Rsrc, address Unaligned Store Word y 2.10 Data Movement Instructions Move the contents of Rsrc to Rdest. move Rdest, Rsrc Move y The multiply and divide unit produces its result in two additional registers, hi and lo. These instructions move values to and from these registers. The multiply, divide, and remainder instructions described above are pseudoinstructions that make it appear as if this unit operates on the general registers and detect error conditions such as divide by zero or over ow. mfhi Rdest mflo Rdest Move the contents of the hi (lo) register to register Rdest. Move From hi Move From lo 18 Move the contents register Rdest to the hi (lo) register. mthi Rdest mtlo Rdest Move To hi Move To lo Coprocessors have their own register sets. These instructions move values between these registers and the CPU's registers. mfcz Rdest, CPsrc Move From Coprocessor Move the contents of coprocessor 's register CPsrc to CPU register Rdest. z z Move Double From Coprocessor 1 y Move the contents of oating point registers FRsrc1 and FRsrc1 + 1 to CPU registers Rdest and Rdest + 1. mfc1.d Rdest, FRsrc1 mtcz Rsrc, CPdest Move To Coprocessor z Move the contents of CPU register Rsrc to coprocessor z 's register CPdest. The MIPS has a oating point coprocessor (numbered 1) that operates on single precision (32bit) and double precision (64-bit) oating point numbers. This coprocessor has its own registers, which are numbered $f0{$f31. Because these registers are only 32-bits wide, two of them are required to hold doubles. To simplify matters, oating point operations only use even-numbered registers|including instructions that operate on single oats. Values are moved in or out of these registers a word (32-bits) at a time by lwc1, swc1, mtc1, and mfc1 instructions described above or by the l.s, l.d, s.s, and s.d pseudoinstructions described below. The ag set by oating point comparison operations is read by the CPU with its bc1t and bc1f instructions. In all instructions below, FRdest, FRsrc1, FRsrc2, and FRsrc are oating point registers (e.g., $f2). Floating Point Absolute Value Double Floating Point Absolute Value Single Compute the absolute value of the oating oat double (single) in register FRsrc and put it in register FRdest. abs.d FRdest, FRsrc abs.s FRdest, FRsrc 2.11 Floating Point Instructions Floating Point Addition Double Floating Point Addition Single Compute the sum of the oating oat doubles (singles) in registers FRsrc1 and FRsrc2 and put it in register FRdest. add.d FRdest, FRsrc1, FRsrc2 add.s FRdest, FRsrc1, FRsrc2 c.eq.d FRsrc1, FRsrc2 c.eq.s FRsrc1, FRsrc2 Compare Equal Double Compare Equal Single Compare the oating point double in register FRsrc1 against the one in FRsrc2 and set the oating point condition ag true if they are equal. Compare Less Than Equal Double Compare Less Than Equal Single Compare the oating point double in register FRsrc1 against the one in FRsrc2 and set the oating point condition ag true if the rst is less than or equal to the second. c.le.d FRsrc1, FRsrc2 c.le.s FRsrc1, FRsrc2 19 Compare Less Than Double Compare Less Than Single Compare the oating point double in register FRsrc1 against the one in FRsrc2 and set the condition ag true if the rst is less than the second. c.lt.d FRsrc1, FRsrc2 c.lt.s FRsrc1, FRsrc2 Convert Single to Double Convert Integer to Double Convert the single precision oating point number or integer in register FRsrc to a double precision number and put it in register FRdest. cvt.d.s FRdest, FRsrc cvt.d.w FRdest, FRsrc Convert Double to Single Convert Integer to Single Convert the double precision oating point number or integer in register FRsrc to a single precision number and put it in register FRdest. cvt.s.d FRdest, FRsrc cvt.s.w FRdest, FRsrc Convert Double to Integer Convert Single to Integer Convert the double or single precision oating point number in register FRsrc to an integer and put it in register FRdest. cvt.w.d FRdest, FRsrc cvt.w.s FRdest, FRsrc Floating Point Divide Double Floating Point Divide Single Compute the quotient of the oating oat doubles (singles) in registers FRsrc1 and FRsrc2 and put it in register FRdest. div.d FRdest, FRsrc1, FRsrc2 div.s FRdest, FRsrc1, FRsrc2 Load Floating Point Double y Load Floating Point Single y Load the oating oat double (single) at address into register FRdest. l.d FRdest, address l.s FRdest, address mov.d FRdest, FRsrc mov.s FRdest, FRsrc Move Floating Point Double Move Floating Point Single Move the oating oat double (single) from register FRsrc to register FRdest. Floating Point Multiply Double Floating Point Multiply Single Compute the product of the oating oat doubles (singles) in registers FRsrc1 and FRsrc2 and put it in register FRdest. mul.d FRdest, FRsrc1, FRsrc2 mul.s FRdest, FRsrc1, FRsrc2 neg.d FRdest, FRsrc neg.s FRdest, FRsrc Negate Double Negate Single Negate the oating point double (single) in register FRsrc and put it in register FRdest. Store Floating Point Double y Store Floating Point Single y Store the oating oat double (single) in register FRdest at address. s.d FRdest, address s.s FRdest, address sub.d FRdest, FRsrc1, FRsrc2 sub.s FRdest, FRsrc1, FRsrc2 Floating Point Subtract Double Floating Point Subtract Single Compute the di erence of the oating oat doubles (singles) in registers FRsrc1 and FRsrc2 and put it in register FRdest. 20 0x7fffffff Stack Segment Data Segment Text Segment 0x400000 Reserved Figure 5: Layout of memory. 2.12 Exception and Trap Instructions rfe Restore the Status register. syscall Return From Exception System Call Register $v0 contains the number of the system call (see Table 1) provided by SPIM. Cause exception . Exception 1 is reserved for the debugger. n break n Break No operation nop Do nothing. 3 Memory Usage The organization of memory in MIPS systems is conventional. A program's address space is composed of three parts (see Figure 5). At the bottom of the user address space (0x400000) is the text segment, which holds the instructions for a program. Above the text segment is the data segment (starting at 0x10000000), which is divided into two parts. The static data portion contains objects whose size and address are known to the 21 ... $fp argument 6 argument 5 arguments 1-4 . . saved registers . . . . . . local variables . . . . . . dynamic area . . . $sp memory addresses Figure 6: Layout of a stack frame. The frame pointer points just below the last argument passed on the stack. The stack pointer points to the last word in the frame. compiler and linker. Immediately above these objects is dynamic data. As a program allocates space dynamically (i.e., by malloc), the sbrk system call moves the top of the data segment up. The program stack resides at the top of the address space (0x7 f). It grows down, towards the data segment. 4 Calling Convention The calling convention described in this section is the one used by gcc , not the native MIPS compiler, which uses a more complex convention that is slightly faster. Figure 6 shows a diagram of a stack frame. A frame consists of the memory between the frame pointer ($fp), which points to the word immediately after the last argument passed on the stack, and the stack pointer ($sp), which points to the last word in the frame. As typical of Unix systems, the stack grows down from higher memory addresses, so the frame pointer is above stack pointer. The following steps are necessary to e ect a call: 1. Pass the arguments. By convention, the rst four arguments are passed in registers $a0{ $a3 (though simpler compilers may choose to ignore this convention and pass all arguments via the stack). The remaining arguments are pushed on the stack. 2. Save the caller-saved registers. This includes registers $t0{$t9, if they contain live values at the call site. 22 3. Execute a jal instruction. Within the called routine, the following steps are necessary: 1. Establish the stack frame by subtracting the frame size from the stack pointer. 2. Save the callee-saved registers in the frame. Register $fp is always saved. Register $ra needs to be saved if the routine itself makes calls. Any of the registers $s0{$s7 that are used by the callee need to be saved. 3. Establish the frame pointer by adding the stack frame size - 4 to the address in $sp. Finally, to return from a call, a function places the returned value into $v0 and executes the following steps: 1. Restore any callee-saved registers that were saved upon entry (including the frame pointer $fp). 2. Pop the stack frame by adding the frame size to $sp. 3. Return by jumping to the address in register $ra. 5 Input and Output In addition to simulating the basic operation of the CPU and operating system, SPIM also simulates a memory-mapped terminal connected to the machine. When a program is \running," SPIM connects its own terminal (or a separate console window in xspim) to the processor. The program can read characters that you type while the processor is running. Similarly, if SPIM executes instructions to write characters to the terminal, the characters will appear on SPIM's terminal or console window. One exception to this rule is control-C: it is not passed to the processor, but instead causes SPIM to stop simulating and return to command mode. When the processor stops executing (for example, because you typed control-C or because the machine hit a breakpoint), the terminal is reconnected to SPIM so you can type SPIM commands. To use memory-mapped IO, spim or xspim must be started with the -mapped io ag. The terminal device consists of two independent units: a receiver and a transmitter . The receiver unit reads characters from the keyboard as they are typed. The transmitter unit writes characters to the terminal's display. The two units are completely independent. This means, for example, that characters typed at the keyboard are not automatically \echoed" on the display. Instead, the processor must get an input character from the receiver and re-transmit it to echo it. The processor accesses the terminal using four memory-mapped device registers, as shown in Figure 7. \Memory-mapped" means that each register appears as a special memory location. The Receiver Control Register is at location 0x 0000 only two of its bits are actually used. Bit 0 is called \ready": if it is one it means that a character has arrived from the keyboard but has not yet been read from the receiver data register. The ready bit is read-only: attempts to write it are ignored. The ready bit changes automatically from zero to one when a character is typed at the keyboard, and it changes automatically from one to zero when the character is read from the receiver data register. Bit one of the Receiver Control Register is \interrupt enable". This bit may be both read and written by the processor. The interrupt enable is initially zero. If it is set to one by the 23 Unused Receiver Control (0xffff0000) 1 1 Interrupt Enable Unused Receiver Data (0xffff0004) 8 Ready Received Byte Unused Transmitter Control (0xffff0008) 1 1 Interrupt Enable Ready Unused Transmitter Data (0xffff000c) 8 Transmitted Byte Figure 7: The terminal is controlled by four device registers, each of which appears as a special memory location at the given address. Only a few bits of the registers are actually used: the others always read as zeroes and are ignored on writes. 24 processor, an interrupt is requested by the terminal on level zero whenever the ready bit is one. For the interrupt actually to be received by the processor, interrupts must be enabled in the status register of the system coprocessor (see Section 2). Other bits of the Receiver Control Register are unused: they always read as zeroes and are ignored in writes. The second terminal device register is the Receiver Data Register (at address 0x 0004). The low-order eight bits of this register contain the last character typed on the keyboard, and all the other bits contain zeroes. This register is read-only and only changes value when a new character is typed on the keyboard. Reading the Receiver Data Register causes the ready bit in the Receiver Control Register to be reset to zero. The third terminal device register is the Transmitter Control Register (at address 0x 0008). Only the low-order two bits of this register are used, and they behave much like the corresponding bits of the Receiver Control Register. Bit 0 is called \ready" and is read-only. If it is one it means the transmitter is ready to accept a new character for output. If it is zero it means the transmitter is still busy outputting the previous character given to it. Bit one is \interrupt enable" it is readable and writable. If it is set to one, then an interrupt will be requested on level one whenever the ready bit is one. The nal device register is the Transmitter Data Register (at address 0x 000c). When it is written, the low-order eight bits are taken as an ASCII character to output to the display. When the Transmitter Data Register is written, the ready bit in the Transmitter Control Register will be reset to zero. The bit will stay zero until enough time has elapsed to transmit the character to the terminal then the ready bit will be set back to one again. The Transmitter Data Register should only be written when the ready bit of the Transmitter Control Register is one if the transmitter isn't ready then writes to the Transmitter Data Register are ignored (the write appears to succeed but the character will not be output). In real computers it takes time to send characters over the serial lines that connect terminals to computers. These time lags are simulated by SPIM. For example, after the transmitter starts transmitting a character, the transmitter's ready bit will become zero for a while. SPIM measures this time in instructions executed, not in real clock time. This means that the transmitter will not become ready again until the processor has executed a certain number of instructions. If you stop the machine and look at the ready bit using SPIM, it will not change. However, if you let the machine run then the bit will eventually change back to one. 25 ...
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