How to implement CALL in LMM ?

Bean · 2010-02-03 19:34

I've just been playing around with LMM and was trying to figure out how to implement CALL #label.

Here is what I've come up with, but it seems like alot of instructions. And requires 2 temp variables.

Is there an easier way ?

  CALL #label ->      rdlong temp,pc  ' temp = address of "LONG 0-0"
                      LONG @label_RET
                      mov temp2,pc    ' pc=next instruction
                      add temp2,#16   ' add 4 instructions
                      wrlong pc,temp  ' set return address
                      rdlong pc,pc    ' jump to label
                      LONG @label
                      ...
                    label
                      ...
   RET        ->      rdlong pc,pc    ' jump back to caller
                    label_RET
                      LONG 0-0

I'm not concerned about handling a CALL in PASM. I'll just generate whatever code is needed to replace CALL.

Bean
·

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Ale · 2010-02-03 19:51

I use this:

krnl_lcall      mov     krnl_lr,krnl_pc ' save PC in link register
                add     krnl_lr,#4
                rdlong  krnl_pc,krnl_pc ' address of destination
                jmp     #krnl_fetch

krnl_ljmp       rdlong  krnl_pc,krnl_pc   ' reads address of destination
                jmp     #krnl_fetch

' LMM code follows

                jmp     #krnl_lcall
                long     routine_to_call

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Bean · 2010-02-03 20:40

Ale,
I don't see how nested subroutines would be handled ? Is there something I'm missing ?

Bean

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Use BASIC on the Propeller with the speed of assembly language.

PropBASIC thread http://forums.parallax.com/showthread.php?p=867134

Bill Henning · 2010-02-03 20:54

You have to maintain a software based return stack, either in-cog (faster) or in hub memory (slower, but larger stack possible)

Bean said...
Ale,
I don't see how nested subroutines would be handled ? Is there something I'm missing ?

Bean

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RossH · 2010-02-03 22:25

Hi Bean,

As Bill mentioned, you need a call stack - here is how Catalina does it (the stack is in hub RAM and only requires one or two additonal instructions to manage):

f_ret
       rdlong PC,SP         ' read the PC
       add    SP,#4         ' increment the SP
       jmp    #LMM_loop     ' execute the next instruction
'
' f_call - call the routine at the address pointed to by the PC (increment the PC, save return address on stack)
'
f_call                    
       rdlong RI,PC         ' get the address to call
       add    PC,#4         ' increment the PC (this is the return address)
       sub    SP,#4         ' decrement the SP
       wrlong PC,SP         ' save current PC to stack
       mov    PC,RI         ' get the address to jump to
       jmp    #LMM_loop     ' execute next instruction

Then a call and a return might look like:

my_function
       ... do_stuff_here ...
       jmp #f_ret

my_main
       ... do stuff ...
       jmp #f_call
       long @my_function
       ... do_more_stuff ...

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How to implement CALL in LMM ?

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