I'm trying to determine what various flags should be set to (carry, sign, zero, overflow) after the following operation:
mov ax, 7ff0h; ax register is now 7ff0h
add al, 10h;
I'm confused as to how assembly handles this. since we are adding to just the al portion of the register which contains f0h, f0h + 10h gives 100h. Does this set the overflow flag? The carry flag? Neither? I would think the overflow flag would be set, however, another possibility running through my mind is that the program detects this and automatically alters the ah register from 7fh to 80h, making the full ax register 8000h. This would in theory not set the carry flag, but would instead set the sign flag and overflow flag because our overall number 8000h is now negative. could anyone explain how this is handled?
Thanks.
I have played a bit with the debug command (shipped with Intel 32-bit Windows or MS-DOS):
-a 100
1A2F:0100 mov ax, 7ff0
1A2F:0103 add al, 10
1A2F:0105
-t =100 2
AX=7FF0 BX=0000 CX=0000 DX=0000 SP=FFEE BP=0000 SI=0000 DI=0000
DS=1A2F ES=1A2F SS=1A2F CS=1A2F IP=0103 NV UP EI PL NZ NA PO NC
1A2F:0103 0410 ADD AL,10
AX=7F00 BX=0000 CX=0000 DX=0000 SP=FFEE BP=0000 SI=0000 DI=0000
DS=1A2F ES=1A2F SS=1A2F CS=1A2F IP=0105 NV UP EI PL ZR NA PE CY
1A2F:0105 0000 ADD [BX+SI],AL DS:0000=CD
-
NV [No Overflow] PL (positive) CY [Carry]