Question about NEAR directive in vasm

[dansalvato]

Hello, I'm new to Amiga ASM and have been exploring options for base-relative addressing. I've taken a liking to the NEAR directive for its implicitness in converting absolute label references into base-relative ones.

The issue I have is that NEAR doesn't seem to support the relative index addressing mode. For example:

NEAR  a5
XREF _LinkerDB

DATA
SomeStruct:
dcb.l  4

CODE
lea     _LinkerDB,a5

; This works
move.l  SomeStruct+4,d0

; This is identical to the above
move.l SomeStruct+4(a5),d0

; This does not work
move.l (SomeStruct,d1.w),d0

; Directly referencing a5 also does not work
move.l SomeStruct(a5,d1.w),d0

; This works
lea    SomeStruct,a0
move.l (a0,d1.w),d0

I don't know much about the linker, but is this a limitation based on how NEAR interacts with the linker? This isn't an issue when using BASEREG, although I think I understand how BASEREG works differently and is a more "simple" conversion at assemble time. But I figured if vasm can correctly assemble these instructions with indirect addressing, there shouldn't be anything stopping it from doing indirect with index.

Any insight on what I might be missing about NEAR, and if I should just use BASEREG instead? Thanks very much in advance.

[dansalvato]

I think I answered my own question—the index addressing mode only supports an 8-bit offset value, but NEAR (and base-relative addressing in general) works on the premise that a 16-bit offset is available to fetch data relative to the base address. So, it makes sense that you wouldn't be able to combine an index with this addressing mode.

[phx]

Originally Posted by Haxxy

I've taken a liking to the NEAR directive for its implicitness in converting absolute label references into base-relative ones.

For anybody who is wondering why the given example doesn't for him, please note that vasm needs either the

-sd

or the

-phxass

option to enable automatic small-data translations. Also, the referenced symbol should be in a small-data section, named "__MERGED".

; Directly referencing a5 also does not work
move.l SomeStruct(a5,d1.w),d0

That's because this is not a typical small-data addressing mode, which usually has the form

(d16,An)

. You can enable small-data for 68020 extended addressing modes with the

-extsd

option, but you still have to explicitely define

SomeStruct

as a 16-bit displacement. For example, these will work and should generate 16-bit small-data relocs (with -extsd):

        move.l  (SomeStruct.w,a5,d1.w),d0
        move.l  ([SomeStruct.w,a5],d1.w),d0

I don't know much about the linker, but is this a limitation based on how NEAR interacts with the linker?

Here it is more an issue of tradition, enforced by the assembler. The linker (at least vlink) could theoretically handle 8-, 16- and 32-bit base-relative relocations. And AmigaDOS defines HUNK_DREL8, HUNK_DREL16 and HUNK_DREL32 for them.

Traditionally, only 16-bit base relative was ever used in Amiga programs. Certainly for a reason. 8-bit leaves you with only a tiny small-data segment of 256 bytes, and there is no

(d8,An)

addressing mode on 68000 anyway. 32-bit gives you no size advantage to absolute addressing and requires 68020.

and if I should just use BASEREG instead?

BASEREG is only an option if you don't have to link your object with other objects. It is local.