SpartaDOS X #17

I might submit a PR with support for SDX binaries, but I'd like to know your opinion about the scope first:

1. Won't accept anyway, other assemblers can do it.
2. Fast Assembler / MADS style blk and smb directives. Relatively easy with small diff surface, limited to SDX.
3. Emit llvm-mos-like ELF or obj65 first, add a linker with SDX target. May end up nice, or never finished rabbit hole.
4. other?

Everything below this line will be updated as the spec is refined.

Summary

Add support for the SpartaDOS X binary format: relocatable code blocks, allocation blocks, symbol imports and exports, with type-checked relocation.

The work starts by moving object-file generation (block headers, gap filling, the trailer written at the end) out of the assembler core into one class per output format. SpartaDOS X is the first new class. ELF (llvm-mos conventions) and o65 are reserved names in this issue: the syntax, the value typing and the class interface are designed so that they can be added later as new classes only, without touching the directives or the evaluator.

Reference: SpartaDOS X Programming Guide V. 4.50, chapter 2 (file format), 3.4 (program extended area), 3.6 (memory indices), 19.1.3.1 (control byte).

        opt 'output=spartadosx'
printf  xrf 'PRINTF'            ; import a system symbol
gets    xrf 'GETS'
bufsz   equ 32

        rel 'text'              ; relocatable block named TEXT, in main memory
hello   xdf '@HELLO'            ; export this location; @ makes it a shell command
        jsr printf
        dta c'Type your name: ',0
        lda whov
        ldx whov+1
        ldy #bufsz
        jsr gets
        bmi quit
        jsr printf
        dta $9b,c'Hello, %s!',$9b,0
quit    rts
whov    dta a(who)              ; relocated automatically

        bss 'bss'               ; allocation-only block named BSS
who     org *+bufsz

Output settings

Depends on named opt settings (#23), the exact semantics may change.

        opt 'output=ataridos'   ; Atari DOS executable, $FFFF signature (default; opt h+)
        opt 'output=raw'        ; plain bytes, no headers (opt f-h-)
        opt 'output=rom'        ; plain bytes, gaps filled with $FF (opt f+h-)
        opt 'output=spartadosx' ; SpartaDOS X executable, $FFFA absolute blocks, sections allowed, no short `opt` syntax

Format capabilities

Under spartadosx, a run word gets a fixup when it points into a section. An ini address must be absolute; an address in a section or an imported symbol is an error (see SDX output).

Directives

Four directives, one string operand each: rel/bss open sections, xrf imports a symbol (XREF, $FFFB) and xdf exports one (XDEF, $FFFC). The operand is parsed with the opt rules: comma-separated items, case-insensitive keys, no spaces. The first item is positional; the rest are key or key=value. The operand is always required. As with every xasm directive, the number of operands is fixed, so there's no bare rel or bss.

All four are errors under raw, rom and ataridos. A label on a rel/bss line gets the section's current offset: 0 for a new section, the length so far for a reopened one.

Section names. The first item is the section's name. It's the key for reopening the section, in every format. The format parses it:

Section names are a namespace of their own. They can't clash with labels or with xrf/xdf names.

Under spartadosx, each name is one $FFFE block:

• Numbering: blocks are numbered from 1 in the order their names first appear, with at most 7 blocks per file, rel and bss together (see "Loader behaviour"). The listing shows the block number on the line that opens the block, so a program that reads the EXTENDED+3 table (guide 3.7) can rely on it.
• Memory: the attribute main ($00, the default), extended ($02) or program ($04, SDX 4.47+). For example, rel 'drv,extended,align=256' gives control byte $42.
• Reopening: it may repeat the attributes or leave them out (rel 'text'). If it gives them, they must match the first opening. Using one name for both a rel and a bss section is an error. A block that an ini flush has already written out can't be reopened; the error suggests using a new name.
• Offsets: a reopened block continues at its current length. * and labels count from the start of the block, whatever part of the source they are in.

This lets code and data from several included sources go into one text block and one bss block, without the included files knowing anything about block numbers.

Attributes:

Any other name, attribute or value is an error. The guide (3.6) says SDX memory indices other than $00, $02 and $04 (plus $40) are reserved with undefined behaviour.

Symbol names under spartadosx: 1 to 8 characters. @ is allowed only as the first character. The other characters are the ones allowed in xasm labels. Names are uppercased and padded with spaces.

• An xdf name exported twice is an error.
• Several xrf labels may import the same name; they share one $FFFB record.
• An xrf that is never referenced writes no record and triggers the unused-label warning (opt u).