flat assembler - [solved] Flat assember G - Macros and address calculation

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ctlaltdel

Joined: 10 Aug 2020
Posts: 4

ctlaltdel 10 Aug 2020, 09:21

Hi all,

I am writing some code for a 'move machine', i.e. a 'one-instruction' CPU that has an instruction format of <src>, <dst> i.e. move byte from 16-bit source address to 16-bit destination address. Memory is byte-addressable. Computation is carried out via pre-computed tables and self-modifying code.

I have a problem with the address calculation of a local label within a macro. Within the 'table_lookup' macro definition, the local label l1 is defined after another macro ('zmgen') is invoked. The 'zmgen' macro is passed l1 as a parameter as it is supposed to overwrite data at this location. 'zmgen' conditionally invokes yet another macro ('pbrk') if the lower 8 bits of the current assembly address is 0x00.
The problem is that it seems the address of l1 is calculated before 'zmgen' and 'pbrk' are assembled, and so that if assembly of the macro starts at 0x??00, the effective address of l1 is incorrect. Code below:

Code:

; insert page break code
        macro   pbrk
                dw      t_ident+12,IP                   ; do jump past this code
                dw      jdest+1,IP+1                    ; do high part of jump
                dw      jdest,IP                        ; do low part of jump
        end macro

; generate one instruction, account for 256-byte page breaks
        macro   zmgen   s,d
                if      ($ and $ff) = 0
                        pbrk
                end if
                P isname s
                if P = 1
                        dw   t_ident+s,d
                else
                        dw   s,d        
                end if                  
        end macro

; table lookup 
        macro   table_lookup    table, offset, dest
                zmgen   offset, l1
                local   l1
l1:
                zmgen   table, dest
        end macro

Is there a way to have l1 calculated after 'zmgen' and 'pbrk' are assembled? Is there a better way?

Thanks in advance

10 Aug 2020, 09:21

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8356
Location: Kraków, Poland

Tomasz Grysztar 10 Aug 2020, 09:58

I see one probable problem here:

Code:

        macro   table_lookup    table, offset, dest
                zmgen   offset, l1
                local   l1
l1:
                zmgen   table, dest
        end macro

The LOCAL directive applies only to uses of "l1" symbol that follow in the macro (similarly to how it works in fasm 1). Therefore your "zmgen offset,l1" line is not affected and refers to the global "l1" symbol, while later uses of "l1" within that macro refer to the local one, a distinct symbol specific to each instance the macro is called.

You should move "local l1" line to the beginning of the macro in order for it to affect all uses of the "l1" name.

10 Aug 2020, 09:58

ctlaltdel

Joined: 10 Aug 2020
Posts: 4

ctlaltdel 10 Aug 2020, 10:07

Actually, it was originally at the beginning of the macro - I moved it to see what effect it would have (nothing useful), and forgot to move it back.

10 Aug 2020, 10:07

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8356
Location: Kraków, Poland

Tomasz Grysztar 10 Aug 2020, 10:10

What value is incorrect then? Can you give a minimal complete source that can be assembled to demonstrate your problem?

10 Aug 2020, 10:10

ctlaltdel

Joined: 10 Aug 2020
Posts: 4

ctlaltdel 11 Aug 2020, 06:07

I'll do a bit more investigation and see if I can come up with a compact demonstration of the problem.

11 Aug 2020, 06:07

ctlaltdel

Joined: 10 Aug 2020
Posts: 4

ctlaltdel 11 Aug 2020, 08:26

Ok, found (and fixed) the problem. It was of course, my fault, not the assembler's. I overlooked the fact that the 'zmgen' macro invokes the 'pbrk' macro on a byte boundary, so an earlier instruction that is supposed to overwrite part of the source address of the 'zmgen' instruction actually overwrites part of the 'pbrk' code instead, causing the wheels to fly off. Oops. Added conditional assembly of an offset and all is good.

Anyway, thanks for a great assembler, Tomasz!

11 Aug 2020, 08:26

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