I want to convert simple loops in high-level languages into assembly language (for emu8086) say, I have this code:

 for(int x = 0; x<=3; x++) { //Do something! }

or

 int x=1; do{ //Do something! } while(x==1)

or

 while(x==1){ //Do something }

How do I do this in emu8086?

2 Answers

For-loops:

For-loop in C:

for(int x = 0; x<=3; x++)
{ //Do something!
}

The same loop in 8086 assembler:

 xor cx,cx ; cx-register is the counter, set to 0
loop1 nop ; Whatever you wanna do goes here, should not change cx inc cx ; Increment cmp cx,3 ; Compare cx to the limit jle loop1 ; Loop while less or equal

That is the loop if you need to access your index (cx). If you just wanna to something 0-3=4 times but you do not need the index, this would be easier:

 mov cx,4 ; 4 iterations
loop1 nop ; Whatever you wanna do goes here, should not change cx loop loop1 ; loop instruction decrements cx and jumps to label if not 0

If you just want to perform a very simple instruction a constant amount of times, you could also use an assembler-directive which will just hardcore that instruction

times 4 nop

Do-while-loops

Do-while-loop in C:

int x=1;
do{ //Do something!
}
while(x==1)

The same loop in assembler:

 mov ax,1
loop1 nop ; Whatever you wanna do goes here cmp ax,1 ; Check wether cx is 1 je loop1 ; And loop if equal

While-loops

While-loop in C:

while(x==1){ //Do something
}

The same loop in assembler:

 jmp loop1 ; Jump to condition first
cloop1 nop ; Execute the content of the loop
loop1 cmp ax,1 ; Check the condition je cloop1 ; Jump to content of the loop if met

For the for-loops you should take the cx-register because it is pretty much standard. For the other loop conditions you can take a register of your liking. Of course replace the no-operation instruction with all the instructions you wanna perform in the loop.

Do{ AX = 0 AX = AX + 5 BX = 0 BX= BX+AX
} While( AX != BX)

Do while loop always checks the loop the condition at the end of each iteration.