Write a program that calculates the expression Res = (A + B) – (C + D), using re

Question

Write a program that calculates the expression Res = (A + B) – (C + D), using registers and variables. Several steps are suggested:

In the data segment, define variables ordered by: varA, varB, varC, varD, and Res with some integers initialized

In the code segment

calculate (A + B) – (C + D)

save the result in Res

You can copy the following to fill the blanks with your implementations. Please do not use more than six instructions.

- If you can use registers, don’t use memory variables. E.g., if available, use EAX,EBX,ECX,EDX, ESI,EDI, instead of creating var1, var2, etc..

- If possible, use less instructions (no more than 6 instructions), E.g., for

                MOV ebx, var1

                ADD eax, ebx

            simply

                ADD eax, var1

- Use an instruction with less bytes. E.g., both can move EAX value to var1

                MOV var1, eax

                XCHG var1, eax

        If only want to move, logically use MOV, also because here MOV takes 5 bytes machine code but XCHG 6.

- If you can use symbolic constant, don’t use memory variable. E.g., for make an array count for

            ary BYTE 1,2,3,4,5,6,7,8,9

      use constant

            aryCount = ($ - myArray)

      don’t use variable

            aryCount BYTE ($ - myArray)

When finished, test your program in debugger. If you define DWORD varA as 10, varB 20, varC 30, and varD 40, Res should be -40

  ---------------------- CODE ----------------------

  .386  .model flat,stdcall  .stack 4096  ExitProcess proto,dwExitCode:dword    .data  ; define variables: varA, varB, varC, varD, and Res  ; ... ...    .code  main1 proc  ; calculate Res = (A + B) – (C + D)     ; ... ...     ; calculate (A + B) – (C + D)     ; ... ...     ; save the result in Res       invoke ExitProcess,0  main1 endp  end main1  

Explanation / Answer

---------------------- CODE ----------------------

.386

.model flat,stdcall

.stack 4096

ExitProcess proto,dwExitCode:dword

.data

; define variables: varA, varB, varC, varD, and Res

; ... ...

varA DB 10

varB DB 20

varC DB 30

varD DB 40

Res DB ?

.code

main1 proc

; calculate Res = (A + B) – (C + D)

mov eax,0

mov ebx,0

; calculate (A + B) – (C + D)

add eax,[varA]

add eax,[varB]

add ebx,[varC]

add ebx,[varD]

sub eax,ebx

; save the result in Res

mov [Res],eax

invoke ExitProcess,0

main1 endp

end main1

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