Because SI is not calculated right. This is right (I believe, never wrote a bootloader):
[ORG 0x7C00]
start:
JMP 0x0000:setCS ; CS is now 0
setCS:
XOR AX, AX
MOV DS, AX
MOV ES, AX
Better not set FS and GS at this time because your code might be running on a 286 or 8086, without any FS or GS at all.
HELP!!! My bootsector is WAY to big!!!
-
JAAman
RE:Oops:7C00=07C0
yes but you cannot start with that JMP if you want to be FAT12 complient
better is to leave CS alone and only use conditional jumps till you jump to your second-stage then use full sector:offset to force the offset you want and use either 7C0 or 0000 for DS
beware: if you set DS=07C0 then you MUST use ORG 0000 NOT ORG 7C00
because ORG tells your assembler what to add to the offset when calculating the location of variables so you want that to be the offset in DS that your code starts
I would use DS=0000 and ORG 7C00 because it gives access to all important variables in BIOS space and your code and space to work between
better is to leave CS alone and only use conditional jumps till you jump to your second-stage then use full sector:offset to force the offset you want and use either 7C0 or 0000 for DS
beware: if you set DS=07C0 then you MUST use ORG 0000 NOT ORG 7C00
because ORG tells your assembler what to add to the offset when calculating the location of variables so you want that to be the offset in DS that your code starts
I would use DS=0000 and ORG 7C00 because it gives access to all important variables in BIOS space and your code and space to work between