vasm - Illegal relocation for basereg on merged BSS

[dansalvato]

            XREF        _LinkerDB
            NEAR        a4

            SECTION     __MERGED,DATA

_DataStart:
SomeData:   dcb.b       64
            
            SECTION     __MERGED,BSS

SomeBSS:    dcb.b       64

            BASEREG     _DataStart,a3

            CODE

_start:
            movem.l     d0-a6,-(sp)
            lea         _DataStart,a3
            move.l      SomeData(a3),d0   ; This works
            move.l      SomeBSS(a3),d0    ; Illegal relocation

See above. I'm using "-sd" on vasm to merge data sections, but the BASEREG directive still fails when referencing a symbol in the BSS.

The NEAR directive handles this just fine, and I'm curious what the difference is. When using NEAR, do all data references turn into relocation entries that resolve at load time? Or does vlink resolve all the base-relative offsets ahead of time, with only _LinkerDB requiring relocation?

Basically, I'm wondering if vasm can somehow understand that the __MERGED data and BSS sections will be in contiguous memory, and allow BASEREG to flow from one into the other. But maybe I'm misunderstanding something, and "-sd" does not in fact guarantee that they are contiguous.

I appreciate any insight.

[Don_Adan]

I dont use vasm, but bss section can contains only zero/null bytes. Maybe dont likes BASEREG inside. You can change section from BSS to DATA and check.

[Thomas Richter]

Originally Posted by dansalvato

The NEAR directive handles this just fine, and I'm curious what the difference is. When using NEAR, do all data references turn into relocation entries that resolve at load time? Or does vlink resolve all the base-relative offsets ahead of time, with only _LinkerDB requiring relocation?

References relative to a base register are (and have to be) resolved at link time. The AmigaDOS loader (aka LoadSeg()) cannot resolve such references.

[SToS]

Solution provided for clean "error free" assembly only, NOT tested for desired behaviour.
Changes mad are in bold.

            XREF        _LinkerDB
            NEAR        a4

            SECTION     __MERGED,DATA

_DataStart:
            RSRESET

SomeData:   rs.b       64
            
            SECTION     __MERGED,BSS

SomeBSS:    rs.b       64

            BASEREG     _DataStart,a3

            CODE

_start:
            movem.l     d0-a6,-(sp)
            lea         _DataStart,a3
            move.l      SomeData(a3),d0   ; This works
            move.l      SomeBSS(a3),d0    ; Illegal relocation

[phx]

Originally Posted by dansalvato

I'm using "-sd" on vasm to merge data sections,

-sd

is not meant to merge sections. Sections named

__MERGED

are always merged by any real AmigaDOS linker (like blink, slink, phxlnk, vlink, etc.), sorted by type (Data first, then BSS). Confusingly, there is also a

-sd

option in vlink, which will indeed merge all Data and BSS sections, regardless of name.

What

-sd

does in vasm is just to enable optimization of

symbol

to

symbol(a4)

, whenever the symbol is from such a small-data section (named

__MERGED

) and the base register is the same as specified by the

near

directive.

It has no relevance in this case.

but the BASEREG directive still fails when referencing a symbol in the BSS.

BASEREG

is a different concept than regular small-data. It can only access data from the section where the base-symbol (here

_DataStart

) is defined in, via the given base register.

When using NEAR, do all data references turn into relocation entries that resolve at load time?

Regular small data (

NEAR

directive) creates relocations entries, which the linker will resolve.

BASEREG

only works locally, with a single section, and does not create relocations.

Basically, I'm wondering if vasm can somehow understand that the __MERGED data and BSS sections will be in contiguous memory, and allow BASEREG to flow from one into the other.

No. The assembler does not merge sections. That's a task for the linker.

[dansalvato]

Originally Posted by phx

-sd

is not meant to merge sections. Sections named

__MERGED

are always merged by any real AmigaDOS linker (like blink, slink, phxlnk, vlink, etc.), sorted by type (Data first, then BSS). Confusingly, there is also a

-sd

option in vlink, which will indeed merge all Data and BSS sections, regardless of name.

Thanks for clearing this up for me. I really appreciate your availability in answering questions like this. Indeed, I think some of the naming confused me, between

-sd

being an option in both vasm and vlink, and the name

__MERGED

being used for both Data and BSS sections. I understand now that vasm has no visibility/understanding that the linker will merge the sections.

There are a couple more things I'm wondering about, but it's part of a bigger question with more dedicated discussion, so I will create a new thread. The main question in this thread has been answered—thank you!