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HW5 solution

  1. (22 points) As discussed on pages 132–134 (Section 2.12 "Assembler"), pseudoinstructions are not part of the RISC-V instruction set, but often appear in RISC-V programs. For each pseudoinstruction in the following table, produce a minimal sequence of core RISC-V instructions to accomplish the same thing.

    You may need to use x31 for some of the sequences, which the CSSE232-assembler treats as an "assembler temporary" register. In the following tables, BIG refers to a specific number that requires 32 bits to represent and SMALL to a number that can fit in 11 bits. You may need to refer to portions of the immediates using indices (e.g. BIG[11:0]). Unless otherwise specified assume all integers are signed.

    Note: pseudoinstructions can have the same name as core instructions but use different operands than the core instructions.

    Pseudoinstruction What it accomplishes Actual instructions
    move x5, x6 x5 = x6 add x5, x6, x0
    clear x5 x5 = 0 add x5, x0, x0
    li x5, SMALL x5 = SMALL addi x5, x0, SMALL
    li x6, BIG x6 =big
    assume
    BIG[11] = 0    		 lui x6 BIG[31:12]
    addi x6, x6, BIG[11:0]
    li x6, BIG x6 =big
    assume
    BIG[11] = 1    		 lui x6 BIG[31:12]+1
    addi x6, x6, BIG[11:0]
    beq x5, SMALL, L if (x5 == SMALL) goto L
    careful...    		 addi x31, x0, SMALL
    beq x31, x5, L
    beq x6, BIG, L if (x6 == big) goto L
    assume
    BIG[11] = 0    		 lui x31, BIG[31:12]
    addi x31 x31 BIG[11:0]
    beq x31 x6 L
Pseudoinstruction What it accomplishes Actual instructions
ble x9, x11, L if (x9 <= x11) goto L bge x11, x9, L
bgt x9, x11, L if (x9 > x11) goto L blt x11, x9, L
bge x9, x11, L if (x9 >= x11) goto L
solve this without using bge/blt		 slt x31, x9, x11
beq x31, x0, L
addi x6, x13, BIG x6 = x13 + big
careful...		 lui x31, BIG[31:12] + BIG[11]
addi x31, x31, BIG[11:0]
add x6, x13, x31
lw x11, BIG(x13) x11 = Mem[x13 + big] lui x31, BIG[31:12] + BIG[11]
addi x31, x31, BIG[11:0]
add x31, x31, x13
lw x11, 0(x31)