Conversion of Z80 code to TMS9900

[+TheBF]

Just now, Lee Stewart said:

 

should be

*
* POP
*
			MOV *SP+,Rn

 

...lee

Thank you! 

 

[apersson850]

Yes, these two/one instruction(s) are what I typically use for PUSH and POP. I prefer letting the stack grow towards lower addresses, since there is auto-increment, but not decrement, in the repertoire. It also means you can copy the value at TOS by just accessing *SP, since the stack pointer always points at top of stack.

If you write an operating system, or utilities for others to use, then you may want to check for stack overflow. If you write a specific program for yourself, and just push return links, not large data structures, to the stack, then it's not worth the overhead. If you do get a stack overflow in such a case, you have either severely underestimated the stack space you need, or you have created some bug which causes values to just PUSH, not POP.

 

Writing macros for the PUSH and POP functions reduce the risk you forget a "+", like in the example above. I frequently used macros for Branch with Link to Stack and ReTurn with Link from Stack, i.e. instructions including the push and branch for a call and pop and branch for a return.

[adel314]

On 2/22/2021 at 1:32 AM, TheBF said:

I have never used the XOP instructions but that seems like a lot of code to do PUSH and POP plus the BLWP/RTWP overhead.

I suppose if the Assembler does not support macros this provides some abstraction. (?)

 

Push can be two instructions and pop can be one instruction on 9900. Could that work on your system?

 

 

Just re-reading your comment on never having used XOP instructions, I thought I would pass my latest use of them on, which might be of interest to the community.   Why I never thought of this before is surprising, and would have saved me months of debugging time over the years, but there you go.

 

For example, if you define an XOP as say DEBUG using code similar to 

DXOP	DEBUG,15

Then, in your programme code that needs to be debugged you can add the following:

  .
  .
  014E   0200 0003      	LI	R0,3
  0152   0201 0006      	LI	R1,6
  0156   8040           	C	R0,R1
  0158   2FE0 0208      	DEBUG	@TRACE_BUF
  015C   8001           	C	R1,R0
  015E   2FE0 0228      	DEBUG	@TRACE_BUF
  0162   0A10           	SLA	R0,1
  0164   2FE0 0248      	DEBUG	@TRACE_BUF
.
. 0200             TRACE_BUF: BSS  36			;Used to store the trace data
.

and the code in DEBUG(XOP) can produce, during execution, something such as this:

PC = 015C  ST = 1004
R0 =0003 R1 =0006 R2 =0088 R3 =0000 R4 =0500 R5 =7285 R6 =8D77 R7 =574D 
R8 =F2C0 R9 =01C6 R10=0204 R11=9044 R12=8000 R13=D411 R14=1C7B R15=0004

PC = 0162  ST = D004
R0 =0003 R1 =0006 R2 =0088 R3 =0000 R4 =0500 R5 =7285 R6 =8D77 R7 =574D 
R8 =F2C0 R9 =01C6 R10=0204 R11=9044 R12=8000 R13=D411 R14=1C7B R15=0004

PC = 0168  ST = C004
R0 =0006 R1 =0006 R2 =0088 R3 =0000 R4 =0500 R5 =7285 R6 =8D77 R7 =574D 
R8 =F2C0 R9 =01C6 R10=0204 R11=9044 R12=8000 R13=D411 R14=1C7B R15=0004

The DEBUG XOP 15 code is simply (the formatting code is left out but if anyone is interested I can post it),

 

                        ;
                        ; 	NOW SAVE THE TRACE DATA
                        ;
                  XOP15:      
  EE30   CECE           	MOV	R14,*R11+			;STORE PROGRAMME COUNTER FIRST
  EE32   CECF           	MOV	R15,*R11+			;STORE STATUS REGISTER
  EE34   0208 0010      	LI	R8,16			    ;16 REGISTERS
  EE38   CEFD     XOP_LOOP:	MOV	*R13+,*R11+			;COPY REGISTERS
  EE3A   0608           	DEC	R8
  EE3C   16FD           	JNE	XOP_LOOP
  EE3E   022D FFE0      	AI	R13,-32			    ;RESTOR WORKSPACE REGISTER LOCATION

 

Edited May 14, 2021 by adel314
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