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Section
05 Part 05 The JSR and RTS Instructions |
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I refuse to go bungee jumping, I came into this world because
of a broken rubber, I'm not leaving because of one.
~Unknown Author |
The JSR
Instruction
JSR Jump to SubRoutine
The destination operand is moved into the PC, the
return address is moved into the stack, the 68k will continue reading at the
location of the destination
operand.
Examples
This
functions almost exactly the same as the JMP instruction. When the instruction is read, the address is
moved into the PC, and that routine is ran next:
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move.w d0,d1 add.w d1,d1 add.w d1,d0 jsr SkipCode add.w d2,d3 asr.w #$04,d0 SkipCode: move.w d0,d2 ...etc |
(See Section 05 Part 02 for details)
The
difference here is that JSR does one extra thing that JMP does not. JSR stores the return location into the
stack, so the 68k can return back to the JSR, and continue.
The RTS
Instruction
RTS ReTurn
from Subroutine
The return
address is loaded out of the stack and put into the PC,
the 68k will continue reading at the return address.
Examples
Heres
an example of JSR and RTS being used together:
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move.w d0,d1 jsr AddNumber move.w #$0020,d1 jsr AddNumber jmp Continue AddNumber: add.w d1,d2 rts Continue: ...etc |
- First, you have move.w d0,d1
which copies a word from d0 to d1.
- Then you have jsr AddNumber, the 68k jumps to AddNumber and stores
the return address into the stack.
- The add.w d1,d2 adds d1 to d2.
- Then rts reloads the return address from the
stack.
- The 68k returns
to move.w
#$0020,d1
(which is directly after the first jsr AddNumber we hit).
- Then move.w #$0020,d1 copies 0020 into d1.
- Then you have another jsr AddNumber, the 68k jumps to AddNumber and stores
the return address into the stack.
- The add.w d1,d2 adds d1 to d2.
- Then rts reloads the return address from the
stack.
- The 68k returns
to jmp Continue
(which is directly after the second jsr AddNumber we hit).
- Then jmp Continue jumps over our AddNumber: subroutine and continues at Continue:.
Heres
a graphical diagram, first JSR/RTS:
Second
JSR/RTS:
