Chapter 5 Exercises

5-16 A swap procedure can exchange two elements (pointed to by ESI and EDI) of an array using

xchg EAX, [EDI]

xchg EAX, [ESI]

xchg EAX, [EDI]

The above code preserves the contents of the EAX register. This code requires six memory accesses. Can we do better than this in terms of the number of memory accesses if we save and restore the EAX using push and pop stack operations?

5-17 Verify that the following procedure is equivalent to the string_length procedure in section 5.10 which procedure is better and why?

string_length1:

push EBX

sub AX,AX

repeat:

cmp [EBX], word 0

je done

inc AX

inc EBX

jmp repeat

done:

pop EBX

ret

;String length procedure MODULE2.ASM

; Objective: To write a procedure to compute string length of a NULL-terminated string.

; Input: String pointer in EBX register.

; Output: Returns string length in AX.

%include "io.mac"

.CODE

global string_length

string_length:

; all registers except AX are preserved

push ESI ; save ESI

mov ESI,EBX ; ESI = string pointer

repeat:

cmp byte [ESI],0 ; is it NULL?

je done ; if so, done

inc ESI ; else, move to next character

jmp repeat ; and repeat

done:

sub ESI,EBX ; compute string length

mov AX,SI ; return string length in AX

pop ESI ; restore ESI

ret