Assembly Programming: How to Calculate 3a-4c Using MUL Instruction

[risen375]

Im having trouble with this program. It is suppose to calculate 3


call getPos ;AX = a (user input)
M1 dw ?
mov M1, AX ;M1 = a
call crlf
call getPos ;AX = b (user input)
M2 dw ?
mov M2, AX ;M2 = b
call crlf
call getPos ;AX = c (user input)
M3 dw ?
mov M3, AX ;M3 = c
call crlf
mov BX, 2 ;BX = 2
mov CX, 3 ;CX = 3
mov DX, 4 ;DX = 4
mov AX, M1 ;AX = a
mul CX ;AX= 3*a
mov SI, AX ;SI = 3a
mov AX, M3 ;AX = M3
mul DX ;AX = 4*c
mov DI, AX ;DI = 4c
sub SI, DI ;3a-4c
mov AX, SI ;AX = 3a-4c
call putPos ;the sum (being in AX) is displayed
mov ah, 04c
int 021
include ioSubs.inc

 

[chiro]

Hey risen375 and welcome to the forums.

Try changing MOV SI,AX to MOV SI,CX. Also try changing MOV DI,AX to MOD DI,DX.

If I recall correctly, the first operand is the register where something is actually stored to not the second one.

 

[rcgldr]

You should not put data in the middle of your program. If you're going to have code and data in the same segment, the data usually goes after the code. Another problem with your program is that the result of a multiply ends up in DX:AX as a 32 bit values, which is probably setting DX to zero every time you do a MUL. Your code could look like this.

call getpos ; get a
mov m1,ax
...
mov ax,3 ; ax=3a
mul m1
...
mov ah, 04ch ;exit program
int 21h

m1 dw ? ;program data
m2 dw ?
m3 dw ?