Table Branching in assembly

tekochip

I would like to have my code branch based upon a value passed to it, but I haven't found a way for the assembler to resolve the addresses.· Something like this:


············· add······ jumpPtr, par
············· movd····· :dest, JumpPtr
············· nop
:dest······ jmp······ jumpPtr



StringBuildUp
············ nop
·············nop


StringFill
············ nop
············ nop


JumpTable
············· long····· StringBuildUp
············· long····· StringFill
jumpPtr
············ long······ JumpTable
·

tekochip

Got it, I needed one more level of indirection:

add jumpPtr, par 'Grab the offset
movs :dest, jumpPtr 'Pointing at
nop
:dest
movs :dest1, jumpPtr 'now what I'm pointing at
nop
:dest1 jmp jumpPtr

Kaio

Chip,

you could have a look at file AsmDebug.spin of POD, where I pass the command to jump to the related subroutine. I have used a jump table for this while it is the easiest way and you can save memory which is otherwise used to get the address from table.

Kaio

I have separated the code which I was using in POD.

                        rdbyte  Command,par             'get command value in range 0 - 3
                        add     Command,#cmdJumpTable
                        movs    :jmpCmd,Command
                        nop
:jmpCmd                 jmp     #0-0                    
cmdJumpTable            jmp     #subroutine0
                        jmp     #subroutine1
                        jmp     #subroutine2
                        jmp     #subroutine3
'-----------------------
subroutine0             nop
'-----------------------
subroutine1             nop
'-----------------------
subroutine2             nop
'-----------------------
subroutine3             nop

Command                 res     1

tekochip

Much cleaner, thanks.

Gary Preston

Have a look at the parallax graphics driver code, it has a jumptable setup using bytes to minimise the space used. It reads a command from PAR then jumps to the address held in the jmp table.