Jmp/Jsr/Bra/Rts/Rti - Unconditionnal jumps, subroutine call and return

Jmp	: Pc = Register
Jsr	: Stack <- Pc ; Pc = Register
Bra	: Pc = Pc + Immediate16Bits
Rts	: Stack -> PC
Rti	: Stack -> StatusRegister ; Stack->Pc

Affected Flags (jmp,jsr, bra,rts)
C	V	N	Z

All these instructions are related to changing the path of execution or calling subroutine. The Jmp instructions causes a direct jump to the specified location. The Jsr instruction is made to call subroutines : it does the same thing as the Jmp instruction but the address following the Jsr instruction is stored in the stack, so an Rts instruction at the end of the subroutine will cause resuming the execution just after the Jsr instruction.

Affected Flags (rti)
C	V	N	Z
X	X	X	X

The Bra instruction is like Jmp but instead of absolute register addressing mode, Bra uses relative immediate addressing mode like all Bxx instructions.
The Rti instruction mean "return from interrupt". This instruction must be used to terminate an interrupt function and resume the regular program execution flow. Compared to the Rts instruction, the Rti instruction will also restore the status register from the stack. This is the only instruction to affect the status flags.

Forms and variants

rx = Register containing the destination address

Form	Effect
jmp (rx)	Jump to location Pc <- Rx
jsr (rx)	Call subroutine Stack <- Pc Pc <- Rx
bra #label	Jump to location (relative) Pc = Pc + 16BitsSignedImmediate
Rts	Return from subroutine (called with jsr) Pc <- Stack
Rti	Return from interrupt StatusRegister <- Stack Pc <- Stack

Instruction encoding

I = Immediate 16 bit value of the relative jump
D = Register number

Instruction	Op Code	Cycles	Encoding
jmp (rx)	1B	6	00011011 0000DDDD
jsr (rx)	5B	14	01011011 0000DDDD
bra #16BitsSignedValue	60	8	01100000 IIIIIIII IIIIIIII
rts	9B	12	10011011
rti	DB	16	11011011

Code Examples

Note: This example is for understanding and can be optimized.

// Example : Calling a subroutine 

start:     move.l #123456,r0      ; Argument 1
           move.l #356464,r1      ; Argument 2
           move.l #subroutine, r2 ; Address of subroutine
           jsr (r2)               ; Call subroutine
           brk ; end of the program

subroutine:add.l r1,r0            ; doing the subroutine function
           rts                    ; return

Jmp/Jsr/Bra/Rts/Rti - Unconditionnal jumps, subroutine call and return

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