hw4soln - Intro. to Computer Architecture CSE 240 Autumn...

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CSE 240 Autumn 2005 DUE: Mon. 10 October 2005 Intro. to Computer Architecture Homework 4 Write your answers on these pages. Additional pages may be attached (with staple) if necessary. Please ensure that your answers are legible. Please show your work. Due at the beginning of class . Total points: 80. 1. [6 Points] Instruction Encoding. Suppose a machine encodes instructions in 32 bits according to the following format. Also, suppose the encoding must accommodate 164 opcodes and 50 registers. OPCODE SR DR IMM (a) What is the minimum number of bits required to represent the OPCODE field? Answer : Seven bits can represent 128 values (2 7 = 128), so (in this case) seven isn’t enough. Eight bits can represent 256 values (2 8 = 256), so we need eight bits to represent 164 values. Unfortunately, we are not making efficient use of these bits. If possible, the ISA designer might want to find a way to get the number of opcodes down to 128, so that opcode may be represented in one fewer bits. (b) What is the minimum number of bits required to represent each of the register fields ( e.g. , DR)? Answer : First, it is important to observe that both register fields ( i.e. , SR, and DR) can name all registers, so they will all be the same size. Five bits isn’t enough, because 2 5 = 32 < 50. Six bits is just enough, because 2 6 = 64 > 50. Heck, why not give this machine 64 registers? It won’t increase the encoding space. (c) What is the greatest number of bits that are left for the IMM field? If the IMM field encodes a 2’s comple- ment integer, what range of values can be represented with these bits? Answer : We know the whole instruction is 32 bits and the opcode consumes eight bits and each register field consumes six bits (for a total of 6 × 2 = 12 bits). We are then left with 32 - 8 - 12 = 12 bits for the immediate field. In 12 bits we can represent 2 12 = 4096 different values, specifically the range of integers from - 2 11 = - 2048 to 2 11 - 1 = 2047. 2. [8 Points] Instruction Processing. The PC, IR, MAR, MDR, and RF (register file) are structures written (and read) in various phases of the instruction processing cycle, depending on the instruction being executed. In each table cell, below, enter the instructions that result in writes to the corresponding structure (row) during the corresponding phase (column) of the instruction processing cycle. For example, place LDR in the PC row of the FETCH column if LDR causes the PC to be written during the FETCH phase. To make this simpler, let’s only consider the following instructions: ADD, LDR, LEA, ST, and TRAP. The textbook doesn’t directly answer this question ( i.e. , you can’t just “look it up”), so your answers will need to be based on your understanding of what should happen in each execution phase. Answer
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hw4soln - Intro. to Computer Architecture CSE 240 Autumn...

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