0. Estra points (20 points): Consider the following generic C comparison functio

Question

0. Estra points (20 points): Consider the following generic C comparison function and its assembly language representation. C code byte comp (byte a, byte b): /a in rdi, b in rsi Assembly code: cmpb setinst %rsi, %rdi %al - Your jobs (fill-in blank): now show how the condition flags will be set with the corresponding SET instruction and the given values of a and b A 5 points set CF ZF SF OF %al setg 47 B. 5 points set CF ZF SF OF %al setl 23 47 C. 5 points CF ZF SF OF %al set 23 23 sete D. 5 points CF ZF SF OF %al set setne 23 47

Explanation / Answer

cmpb instruction subtracts 2nd operand from 1st and sets flags accordingly.

set_instr %al     sets %al to 1 according to the result of flags set by cmpb instruction

ZF is set if arithmetic operation results in 0.

CF is set if there is any carry/borrow

SF is set if result was negative

OF is set if there is an overflow i.e. arithmetic result is not in the range of 8-bit or 16-bit

A

     setg          47      23

Here, 47 >23 therefore CF = 0   ZF = 0 SF = 0   OF = 0   and %al =1

B

    setl            23      47

setl sets %al to 1 if SF != OF after instruction execution

Here, 23<47 therefore sets SF = 1

All other flags will remain 0

%al will be set to 1 as SF != OF

CF = 0   ZF = 0 SF = 1 OF = 0   and %al =1

C)

sete    23    23

Here, 23 -23 = 0 therefore sets ZF to 1 and %al to 1

CF = 0   ZF = 1 SF = 0 OF = 0   and %al =1

D)

setne     23      47

setne sets %al to 1 if ZF = 0

In this instruction, only SF flag will be set. ZF will remain 0 as ZF is set only if arithmetic result of subtraction is 0.

CF = 0   ZF = 0 SF = 1 OF = 0   and %al =1

set a b CF ZF SF OF %al 0 0 0 0 0

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