Pipeline Task

Assume a 5-stage pipelined processor using the standard 5-stages (IF, ID, EX, MEM, WB) that uses no forwarding or stalling circuitry. Rather, you will use the compiler to add no-ops to the code to ensure correct execution.

Note: You can assume that if the processor reads and writes to the same register in a given cycle, the value read out will be the new value that is written in that cycle.

Write your own short program that re-writes the code below including the no-ops that are needed to protect against hazard conditions.

Code to be changed:

# Lab 8 CS10 w9contextswitcher.asm

# This is an example of a clock tick handler to do simple context

switching.

#

#

# This file holds the syscall 100 handler and the interrupt handler

# which allows us to context switch between two programs. We define

# two process control blocks (PCBs) with room to store the registers

# of both programs when they are not running.

#

# syscall 100 takes two arguments: the base address of both programs

# in $a0 and $a1. It will initialize the PCB for each program with

# the base address, enable the clock tick interrupt from the

# Digital Lab Sim, and then context switch into the 1st program.

# In other words, the syscall will not return back to the syscaller.

# Define a process control block for each program

.kdata

p1pcb: .space 20 # Room for a0, v0, t0, t1 and PC (offsets 0, 4, 8,

12, 16)

p2pcb: .space 20 # Ditto

curpcb: .word 0 # Offset to the current PCB, either 0 or 20

saveat: .word 0 # Saved value of $at register

# This is the entry point for both the syscall handler and

# the clock tick interrupt handler.

.ktext 0x80000180

handler:

.set noat

move $k1, $at

sw $k1, saveat # Save $at

.set at

mfc0 $k0, $13 # Copy the Cause register

srl $k0, $k0, 2 # and shift/AND to get the

andi $k0, $k0, 0xf # exact reason for the event.

# We can only

deal with interrupts (0)

# or syscalls

(8). For anything else

# simply call

exit()

beqz $k0, inthandler

beq $k0, 8, syscallhdlr

exit:

li $v0, 10 # Can't deal with it, exit()

syscall

# Here is the system call 100 handler

syscallhdlr:

bne $v0, 100, exit # Exit if it isn't syscall 100

la $k0, p1pcb

sw $a0, 16($k0) # Save the 1st instruction address for

prog1

sw $0, 12($k0) # and fill the other PCB values as

initially 0

sw $0, 8($k0)

sw $0, 4($k0)

sw $0, 0($k0)

la $k0, p2pcb

sw $a1, 16($k0) # Save the 1st instruction address for

prog2

sw $0, 12($k0) # and fill the other PCB values as

initially 0

sw $0, 8($k0)

sw $0, 4($k0)

sw $0, 0($k0)

la $k0, 0xFFFF0013 # Now enable the clock ticks from the

Digial Lab Sim

li $k1, 1

sb $k1, ($k0)

b restore # and context switch into program 1

# Here is the clock tick interrupt handler

inthandler: # First thing, save the program's registers

la $k0, p1pcb

lw $k1, curpcb

add $k0, $k0, $k1 # k0 = p1pcb + curpcb, i.e. base of current PCB

sw $a0, 0($k0) # Save the a0, v0, t0 and t1 registers

sw $v0, 4($k0)

sw $t0, 8($k0)

sw $t1, 12($k0)

mfc0 $k1, $14

sw $k1, 16($k0) # Save the old program counter from the

EPC register

# With that program's state safely

stored away, we can

# switch our understanding of which is

the current program.

lw $k0, curpcb

li $k1, 20

sub $k0, $k1, $k0 # k0 = 20 - curpcb, i.e. 20 => 0, or 0 => 20

sw $k0, curpcb

# Now we know which is the current

program, we can

# restore that program's state

restore: # Restore registers and reset procesor state

la $k0, p1pcb

lw $k1, curpcb

add $k0, $k0, $k1 # k0 = p1pcb + curpcb, i.e. base of current PCB

lw $a0, 0($k0) # Reload the a0, v0, t0 and t1 registers

lw $v0, 4($k0)

lw $t0, 8($k0)

lw $t1, 12($k0)

lw $k1, 16($k0) # Copy the old program counter into the

EPC register

mtc0 $k1, $14

.set noat

lw $k1, saveat # Reload and

move $at, $k1 # restore $at

.set at

mtc0 $0, $13 # Clear Cause register

mfc0 $k0, $12 #

ori $k0, 0x1 # Re-enable interrupts

mtc0 $k0, $12 # in the Status register

eret # and finally return to the old PC