C7
1. Write an algorithm to implement the subtraction operation for two positive
integers in assembly language.
1. Let the numbers be A, B and the operation be A-B
2. Convert A into binary
3. Convert B into binary
4. Compute 2’s complement of B
i. Invert the bits in B using B xor 11111111
ii. Add 1 to B
5. Add A and the 2’s complement of B.
2. Implement your algorithm in the assembly language describe in the machine
language handout. Test your implementation using the SimpleSim simulator.
; Program name : Subtraction using add only
;Programmer : Joe B
;Last Modified : Sep 16 2003
load R1,1 ;1 added for computing 2's complement
load R2,FFh ;mask for flipping the bits
load R3,first_number;
load R4, second_number;
xor R5, R4,R2 ; flip the 0's and 1's in the second number
addi R5,R5,R1 ; add 1 to the flipped bits to get the 2's complement
addi R5,R5,R3 ; add the numbers to obtain A - B
halt
first_number: db 8 ;A in A-B
second_number: db 5 ;B in A-B
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