CODEWITHSUNDEEP
assemblyx86
iProgram
iProgram

Reputation: 6547

Assembly language prints character twice with function call but not when executed directly

I am trying to make my own bootloader written in 16 bit assembly. I am able to print text to a screen using int 10h, AH = 0Eh.

If I run 

MOV AL, 65
MOV AH, 0Eh 
int 10h
HLT

I am successfully able to print A to the screen.

However, if I am to do

MOV AL, 65
CALL printc

HLT

printc:
  MOV AH, 0Eh 
  int 10h 
  RET

I end up getting AA displayed on the screen instead.

Why is this and how would I fix it?

Here is my full code:

BITS 16
ORG 0x7C00

MOV DS, AX
MOV SI, 0x7C00


MOV AL, 65
CALL printc

HLT

printc:
  MOV AH, 0Eh 
  int 10h 
  RET 

times 510-($-$$) db 0x90
dw 0xAA55

Upvotes: 2

Views: 91

Answers (1)

Jester
Jester

Reputation: 58762

To quote the instruction set reference:

HLT Stops instruction execution and places the processor in a HALT state. An enabled interrupt (including NMI and SMI), a debug exception, the BINIT# signal, the INIT# signal, or the RESET# signal will resume execution.

As you can see, an interrupt will resume execution and at least the timer interrupt is usually running, along with other possible sources such as key presses.

Thus, you should add a loop around the HLT to make it go back to sleep.

Upvotes: 3

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