ATmega8不支持JMP指令

Question

现在我正在编写从内存中间开始的bootloader，但在完成之后我需要去主应用程序，想要尝试jmp 0x00 ，但是我的芯片不支持jmp ，我应该如何启动主应用程序？

Answer 1

I would use RJMP :

Relative jump to an address within PC - 2K +1 and PC + 2K (words). In the assembler, labels are used instead of relative operands.

For example:

entry:
   rjmp reset
   .org 512
reset:
    rjmp foo 
   .org 3072
foo:
   rjmp entry

By the way, there are several other jump instructions (RJMP, IJMP, RCALL, ICALL, CALL, RET, RETI etc.) See this relevant discussion .

Answer 2

Well take a look into RET instruction. It returns to previous location, so you can try:

push 0x00
push 0x00
ret

This should work because while entering into any function you push your current location, and RET makes you go back.

As far as I remember ATmege8 has 16-bit address line, but if I'm not right you may need more push 0x00

Answer 3

why not simply use IJMP?

set Z to 0x00 and use IJMP. may be faster than 2xpush and ret

EOR R30, R30  ; clear ZL
EOR R31, R31  ; clear ZH
IJMP          ; set PC to Z

should be 4 cycles and 3 instruction words (6 Bytes program memory)