Stage
OPl rA, rB
rrmovl rA, rB
irmovl V, rB
Fetch
icode: ifun ← M1[PC]
icode: ifun ← M1[PC]
icode: ifun ← M1[PC]
rA:rB ← M1[PC + 1]
rA:rB ← M1[PC + 1]
rA:rB ← M1[PC + 1]
valC ← M4[PC + 2]
Decode
valP ← PC + 2
valP ← PC + 2
valA ← R[rA]
valA ← R[rA]
valP ← PC + 6
valB ← R[rB]
Execute
valE ← valB OP valA
valE ← 0 + valA
valE ← 0 + valC
Set CC
Memory
Write back
R[rB] ← valE
R[rB] ← valE
R[rB] ← valE
PC Update
PC ← valP
PC ← valP
PC ← valP
Figure 4.16 Computations in sequential implementation of Y86 instructions OPl, rrmovl and
irmovl. These instructions compute a value and store the result in a register. The notation icode:ifun
indicates the two components of the instruction byte, while rA:rB indicates the two components of the
register specifier byte. The notation M1[x] indicates accessing (either reading or writing) one byte at
memory location x, while M4[x] indicates accessing four bytes.
Stage
pushl rA
popl rA
Fetch
icode: ifun ← M1[PC]
icode: ifun ← M1[PC]
rA:rB ← M1[PC + 1]
rA:rB ← M1[PC + 1]
valP ← PC + 2
valP ← PC + 2
valA ← R[rA]
valA ← R[%esp]
valB ← R[%esp]
valB ← R[%esp]
Execute
valE ← valB + (-4)
valE ← valB + 4
Memory
M4[valE] ← valA
valM ← M4[valA]
Write back
R[%esp] ← valE
R[%esp] ← valE
Decode
R[rA] ← valM
PC Update
PC ← valP
PC ← valP
Figure 4.18 Computations in sequential implementation of Y86 instructions pushl and popl. These
instructions push and pop the stack.
Homework Problem 4.32
In our example Y86 problems, we encounter many cases in which we want to add a constant value to a
register. This requires first using an irmovl instruction to set a register to the constant, and then an addl
instruction to add this value to the destination register. Suppose we want to add a new instruction iaddl
with the following format:
iaddl V, rB
0
1
C
0
2
8
3
rB
4
5
Byte
V
This instruction adds the constant value V to register rB. Describe the computations performed to implement
this instruction. Use the computations for irmovl and OPl (Figure 4.16) as a guide.
Homework Problem 4.33
The instruction leave can be used to prepare the stack for returning. It is equivalent to the following Y86
code sequence:
1
2
rrmovl %ebp, %esp
popl
%ebp
Set stack pointer to beginning of frame
Restore saved %ebp and set stack ptr to end of caller's frame
Suppose we add this instruction to the Y86 instruction set, using the following encoding:
leave
0
1
D
2
3
4
5
Byte
0
Describe the computations performed to implement this instruction. Use the computations for popl (Figure
4.18) as a guide.
Homework Problem 4.34
The file seq-full.hcl contains the HCL description for SEQ, along with the declaration of a constant
IIADDL having hexadecimal value C, the instruction code for iaddl. Modify the HCL descriptions of the
control logic blocks to implement the iaddl instruction, as described in Homework Problem 4.32. See the
lab material for directions on how to generate a simulator for your solution and how to test it.
Homework Problem 4.35
The file seq-full.hcl also contains the declaration of a constant ILEAVE having hexadecimal value
D, the instruction code for leave, as well as the declaration of a constant REBP having value 7, the register
ID for %ebp . Modify the HCL descriptions of the control logic blocks to implement the leave instruction,
as described in Homework Problem 4.33. See the lab material for directions on how to generate a simulator
for your solution and how to test it.