UNIVERSITY OF CALIFORNIA, DAVIS Department of Electrical and Computer Engineering
EEC 170 Introduction to Computer Architecture Winter 2021
Quiz 2 Solutions
All instructions in this exam, unless otherwise noted, are RISC-V RV32I instructions. This means that, unless otherwise noted, all registers are 32 bits wide on this exam. Recall that if the instruction writes to a register, that register appears first in the instruction. Unless otherwise indicated, you may not use any RISC-V pseudoinstructions on this exam, only actual RISC-V instructions.
1. The SPUR architecture (RISC-IV) contained an instruction called extract. An extract instruction takes a destination register argument (rd), a source register argument (rs), and an immediate argument (imm) (so, extract rd, rs, imm).
extract does the following: dest<07:00> ← one byte from rs selected by imm.
In English, this instruction takes the immth least-significant byte of rs and writes it into the least significant byte of rd, without changing the other bits of rd. Two examples, assuming that the initial values of the two registers are rd = 0xabcdefgh and rs = 0xijklmnop:
•
• (a)
(b)
extract rd, rs, 0 results in rd ← 0xabcdefop extract rd, rs, 2 results in rd ← 0xabcdefkl
(2 points) If this instruction were implemented in RISC-V, what type should it be?
(12 points) Write a minimal RISC-V (RV32I) code segment that imple- ments extract. Note that all registers are 32 bits wide. Use rd for the destination register, rs for the source register (do not change the value of this register), and rt0, rt1, etc. for any temporaries that you need.
You will get some partial credit if you clearly write a short sentence or two that describes your strategy even if your code has errors.
Solution: I-type. Like other I-type instructions, it has 1 register destination, 1 register source, and 1 immediate. Also, it is most similar in function to immediate-using logic instructions like andi or ori, both of which are I-type.
Solution: Clear the low byte of the destination register, shift the correct byte of the source register into the low byte of a temporary, zero out the rest of the bytes of the temporary, and or the low byte of the temporary into the low byte of the destination.
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1 andi rd, rd, -256
2 addi rt0, r0, imm
3 slli rt0, rt0, 3
4 srl rt1, rs, rt0
5
# this is 0xffffff00 and clears the low byte of rd
# put the immediate in a register
# multiply the immediate by 8
# shift the source register right to put the byte
# of interest into the low byte of rt1
6 andi rt1, rt1, 0xff # zero out all but the low byte of rt1
7 or rd,rd,rt1 #putthelowbyteofrt1intothelowbyteofrd
Let’s watch how this works on the second example above:
# rd = 0xabcdefgh, rs = 0xijklmnop # rd = 0xabcdef00
#rt0=imm=2 #rt0=imm*8=16
1
2 andi rd, rd, -256
3 addirt0,r0,imm
4 slli rt0, rt0, 3
5
6 srl rt1, rs, rt0
7 andi rt1, rt1, 0xff
8 or rd, rd, rt1
2. The following opcode map is for the ARM Thumb instruction set, sourced from the ARM7TDMI Technical Reference Manual. It lists 19 different instruction types (anal- ogous to RISC-V’s R-type, I-type, etc.) down the left side and their bit encodings in the table itself. ARM Thumb instructions are 16 bits wide and the opcode map shows how those 16 bits are used for each of the 19 instruction types. Note that any use of R (Rs, Rd, etc.) indicates a register argument. Assume that any arithmetic immediates are unsigned (because they are). The “ALU operation” category includes all logical operations.
For instance, one of the instruction types is “move shifted register”. Instructions in this category always have bits 15–13 set to zero; bits 12–11 encode the specific instruction within this category; and the other instructions are used for two source registers and an immediate (“offset”).
Pay particular attention to:
• How many registers can be addressed per instruction type
• What it means if there are fewer registers for a particular instruction type than you might otherwise expect (hint: think of what x86 does)
• How many bits are available for each register in the instruction encoding
• If a field contains two possibilities (as indicated by a slash, a/b), you can assume
that some variant of the instruction will use one of the possibilities (a) and another Page 2 of 5
#
# rt1 =
# rt1 =
# rd = 0xabcdefkl
(shift left by 3 == multiply by 8)
0x0000ijkl
0x000000kl
Introduction
variant of the same instruction will use the other possibility (b).
Format
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
Move shifted register
Add and subtract
Move, compare, add, and subtract immediate
ALU operation
High register operations and branch exchange
PC-relative load
Load and store with relative offset
Load and store sign-extended byte and halfword
Load and store with immediate offset Load and store halfword SP-relative load and store Load address Add offset to stack pointer Push and pop registers
01
0
0
0
Op
Offset5
Rs
Rd
02
0
0
0
1
1
1
Op
Rn/ offset3
Rs
Rd
03
0
0
1
Op
Rd
Offset8
04
0
1
0
0
0
0
Op
Rs
Rd
05
0
1
0
0
0
1
Op
H1
H2
Rs/Hs
RdHd
06
0
1
0
0
1
Rd
Word8
07
0
1
0
1
L
B
0
Ro
Rb
Rd
08
0
1
0
1
H
S
1
Ro
Rb
Rd
09
0
1
1
B
L
Offset5
Rb
Rd
10
1
0
0
0
L
Offset5
Rb
Rd
11
1
0
0
1
L
Rd
Word8
12
1
0
1
0
SP
Rd
Word8
13
1
0
1
1
0
0
0
0
S
SWord7
14
1
0
1
1
L
1
0
R
Rlist
Multiple load and store
15
1
1
0
0
L
Rb
Rlist
Conditional branch
16
1
1
0
1
Cond
Softset8
Software interrupt
17
1
1
0
1
1
1
1
1
Value8
Unconditional branch
18
1
1
1
0
0
Offset11
Long branch with link
19
1
1
1
1
H
Offset
Format
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
ARM DDI 0210C
(b) (10 points) Write a minimal set of ARM Thumb assembly instructions to perform the following operation:
(a) (2 points) How many differentFrigeugriest1e-r6sThcaumn bbiensatrdudctrieosnsseedt fboyrmants ARM Thumb in- struction?
Solution: Registers are encoded with 3 bits each, so 23 = 8 registers.
Copyright © 2001, 2004 ARM Limited. All rights reserved. 1-21 r1 = (r2 AND (-500 + r3 + r4)) OR r5
where r# indicates a value in register #.
You may only change r1 (you cannot change the value of any other register). You
may assume r1 is initialized to zero.
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The instructional staff believes the opcode map contains all the information you need to solve this problem, but encourages you to explicitly write down any as- sumptions you are making.
The instructional staff understands that you’ve probably never seen the ARM or ARM Thumb instruction sets before. Use RISC-V notation (e.g., add r1, r1, r1) for the assembly code you’re writing, but adhere to any limitations of the ARM Thumb instruction set.
Solution: Weexpectcorrectsolutionstomaketwoimportantrealizationsabout this instruction set that differ from RISC-V:
• Most instructions take only 2 register arguments, except for add/subtract. • The largest immediate available is 8 bits (255).
1 add
2 subi
3 subi
4 and
5 or
r1,r3,r4 #r1=r3+r4
r1, r1, 250 # can’t fit the whole immediate in one instr … r1, r1, 250 # … now r1 = r3+r4-500
r1, r1, r2 # r1 = r2 AND (r3+r4-500)
r1, r1, r5 # r1 = (r2 AND (r3+r4-500)) OR r5
3. Eddie is trying to simplify the RISC-V instruction set. He makes the following sugges- tions. Explain why making the simplifications he suggests are likely to make the RISC-V implementation more complex or hurt performance (or, equivalently, why they are a bad idea).
(a) (2 points) “Having an S-type instruction class seems silly to me. It has all the same information as an I-type instruction. Why not make all S-type instructions I-type instead?”
(b) (2 points) “This idea of having some instructions with 7 total opcode bits, some with 10 (opcode + funct3), and some with 17 (opcode + funct7 + funct3) is really confusing. Since we have enough opcode space to encode all instructions with a uniform 10 bits of opcode per instruction, let’s make all instructions have 10 opcode bits.”
Solution: In all RISC-V instructions, register reads come from registers spec- ified from bits <24:20> and <19:15>. S-type instructions require reading from two registers, one for address and one for data. If we used bits <11:7> to specify a read register, we would have to add additional logic to handle this special case.
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Solution: IfweadoptedEddie’ssolution,U-typeandUJ-typeinstructionclasses (supporting instructions like LUI, AUIPC, JAL, and JALR) would not be able to support as many immediate bits as they do now. Consequently, some of these instructions that currently require all these bits would no longer be able to be expressed in a single instruction and we would have to use multiple instructions to encode them, increasing code size and decreasing performance.
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