Chapter 5 - Chapter 5: The LC-3 The ISA: Overview o The ISA...

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Chapter 5: The LC-3 The ISA: Overview o The ISA specifies all the info about the computer that the software has to be aware of. o 5.1.1: Memory Organization The LC-3 memory has an address space of 2^16 locations, and an addressability of 16 bits. Not all locations are used for memory. Since 16 bits is one word, the LC-3 is word-addressable o 5.1.2: Registers The most common type of temporary storage locations and the one used in the LC3 is the register set. Each register in the set is called a general purpose register (GPR) The number of bits stored in each register is usually one word, or 16 bits. The LC3 specifies 8 GPR’s, each identified with a 3-bit number. They are referred to as R0, R1, R2,…,R7. Register file = register set o 5.1.3: The Instruction Set An instruction is made up of two things Opcode (what the instruction is asking the computer to do) Operands (who the computer is expected to do it to) The instruction set of an ISA is defined by its set of opcodes, data types, and addressing modes. The addressing modes determine where the operands are located. o 5.1.4: Opcodes The LC3 ISA has 15 instructions, each identified by its unique opcode. The opcode is specified by bits [15:12] of the instruction. Since 4 bits are used, 16 opcodes are possible, but 15 opcodes exist. 1101 is unspecified and reserved. 3 different type of instructions Operate process information Data movement instructions move information between memory and the registers and between registers/memory and i/o devices. Control instructions change the sequence of instructions that will be executed o 5.15: Data Types A data type is a representation of information such that the ISA has opcodes that operate on that representation 2’s complement integers o 5.16: Addressing Modes An addressing mode is a mechanism for specifying where the operand is located. An operand can be found in one of three places: in memory, in a register, or as a
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Chapter 5 - Chapter 5: The LC-3 The ISA: Overview o The ISA...

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