Question

(Computer structure)For the MIPS datapath shown below, several lines are marked with "X".

(a) If the line is cut at place 1. Which of the following two instructions will not work? Briefly explain why.

Add $1, $2,$3

SW $1,100($2)

(b)If the line is cut at place 2, which code snippet will not work? Briefly explain. Code snippet 1:

Add $1, $2, $3

Add $4, $1, $1

Code snippet 2:

Add $1, $2,$3

Add $4, $5,$6

(C) If the line is cut at place 3, which code snippet will not work? Briefly explain.

Code snippet 1:

Addi $1, $0,2

Addi $2, $0,2

    Beq $1, $2, exit

Code Snippet 2:

Addi $1, $0,20

Addi $2. $0,10

LW $1,100($2)

1. Stuck et 0 loft of cut 3. Cut here 2. Cut here !

Thanks for your help!

Answer 1

(a)

If the line is cut at place 1

the instruction that will not work is: ADD $1,$2,$3

the instruction that will work is: SW $1,100($2)

reason:

The line is cut at place 1 means cannot write to register file i.e, R-typed and any instruction with writing back to register file will not work.

in the instruction ADD $1,$2,$3 the value in register $2 and $3 is added and written into $1 but here the writing to register doesn't work so the instruction will not work.

(b)

If the line is cut at place 2

the instruction that will not work is:

ADD $1,$2,$3

ADD $4,$1,$1

the instruction that will work is:

ADD $1,$2,$3

ADD $4,$5,$6

reason:

The line is cut at place 2 means forwarding of the first operand fails i.e, the operand output from one instruction cannot become the first operand of other instruction.

ADD $1,$2,$3

ADD $4,$1,$1

In these instructions, $1 is calculated in the first instruction and is forwarded as the first operand in the next instruction so it will not work.

ADD $1,$2,$3

ADD $4,$5,$6

In these instructions, no operand is forwarded as the first operand from instruction 1 so this will work.

(c)

If the line is cut at the place 3

the instruction that will not work is:

ADDI $1,$0,2

ADDI $2,$0,2

BEQ $1,$2, exit

If the line is cut at place 2

the instruction that will work is:

ADDI $1,$0,20

ADDI $2,$0,10

BEQ $1,$2, exit

reason:

The line is cut at place 3 means jumping to branch target does not work i.e, jump to specific branch from instruction is not possible.

ADDI $1,$0,2

ADDI $2,$0,2

BEQ $1,$2, exit

In this segment of code the value of $1 is 2 and of $2 is also 2

now comparing using branch equal $1=$2 so code flow should jump to exit instruction but that does not work when line 3 is cut

ADDI $1,$0,20

ADDI $2,$0,10

BEQ $1,$2, exit

In this segment of code the value of $1 is 20 and of $2 is also 10

now comparing using branch equal $1!=$2 so there is no problem because there is no need to jump to branch.