Question

Given the following array named 'array1':

array1 DWORD 50h, 51h, 52h, 53h

Write instructions to swap the items in array1 (using only MOV and XCHG instruction), so that the array1 become:

53h, 51h, 50h, 52h

( That means the first item in the array is 53h, second item is 51h, third item is 50h, forth item is 52h).

You can use registers to perform the MOV and XCHG operation.  

Write the code to perform above mentioned transformation.

Answer 1

Solution)

Given INPUT : array1 DWORD 50h, 51h, 52h, 53h

Desired or expecting OUTPUT : array1 DWORD 53h, 51h, 50h, 52h

PROCEDURE : utilize registers to execute or perform the MOV and XCHG operations.

Observed information : The desired output can be reached from the given input by :

Swapping the 1st and 4th elements of input array
Swapping the 3rd and 4th elements of the middle array
Note : The 2nd element of the array, ie 51h, go on same.

Based on the above, the code to transforms the given array1 is as follows:

.data

array1 DWORD 50h, 51h, 52h, 53h ; array to be transformed

.code

main proc

mov eva, 0

mov eja, 0

mov ema, 0

mov ena, 0

mov eva, OFFSET array1 ; moves the address of initial element of array1 to eva

mov eja, OFFSET array1 + sizeof array1 - TYPE array1 ; moves the address of end element of array1 to eja

exchangeLoop :

mov ema, [ eva ] ; moves the element in eva to ema

mov ena, [ eja ] ; moves the element in eja to ena

xchg ema, ena ; exchanging the 2 elements

mov [ eva ] , ema ; moves the element in ema to address in eva

mov [ eja ] , ena ; moves the element in ena to address in eja

add eva ; increases eva to take the second element of the array1 from left

add eva ; increases eva to take the third element of the array1 from left

loop exchangeLoop ; invoke exchanging Loop again to exchange the thrird and fourth elements

invoke ExitProcess , 0

main endp

endmain

The array1 now involving the elements in the expected or desired output order : array1 DWORD 53h, 51h, 50h, 52h

This concluded the answer to entire parts of the question along with the nedded explanations.

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