ASM Inserts iDIV

Question

ASM Inserts iDIV

//Pascal

NumX:=-40;

//Assm

MOV     EAX,NumX
MOV     ECX,4
IDIV    ECX
MOV     IntPart, EAX

Why does IntPart return 1073741815?

1

ассемблер delphi lazarus

Author: BigTows, 2016-12-21

Source

1 answers

score 5 · Accepted Answer

Because the value is unsigned.
You have in EAX a converted unsigned 4-byte integer -40, which gives the value FFFFFFD8, or 4294967256 in decimal form.

Divided by 4, we get 3FFFFFF6, or 1073741814 in decimal form.

Here's how your code should look right:

MOV     EAX,NumX
CDQ
MOV     ECX,4
IDIV    ECX
MOV     IntPart, EAX

The signed extension EAX in EDX:EAX is forgotten...