Free SNES assembler

This program reads symbolic 65816 or 65c816 machine code and compiles (assembles) it into a relocatable object file.

The produced object file is binary-compatible with those made with XA65.

The following mnemonics are supported:

adc, and, asl, bcc, bcs, beq, bit, bmi, bne, bpl, bra, brk, brl, bvc, bvs, clc, cld, cli, clv, cmp, cop, cpx, cpy, db , dec, dex, dey, eor, inc, inx, iny, jml, jmp, jsl, jsr, lda, ldx, ldy, lsr, mvn, mvp, nop, ora, pea, pei, per, pha, phb, phd, phk, php, phx, phy, pla, plb, pld, plp, plx, ply, rep, rol, ror, rti, rtl, rts, sbc, sec, sed, sei, sep, sta, stp, stx, sty, stz, tax, tay, tcd, tcs, tdc, trb, tsb, tsc, tsx, txa, txs, txy, tya, tyx, wai, xba, xce

All the standard addressing modes of the 65816 cpu are supported.

• Implied: nop; clc
• Immediate: lda #value; rep #value etc (size may depend on an operand size setting)
• Short relative: bra end
• Long relative: brl end; per end+2
• Direct: lda $12
• Direct indexed: lda $12,x; lda $12,y
• Direct indirect: lda ($12); pei ($12)
• Direct indexed indirect: lda ($12,x)
• Direct indirect indexed: lda ($12),y
• Direct indirect long: lda [$12]
• Direct indirect indexed long: lda [$12],y
• Absolute: lda $1234
• Absolute indexed: lda $1234,x; lda $1234,y
• Absolute long: lda $123456
• Absolute indexed long: lda $123456,x
• Stack-relative: lda $12,s
• Stack-relative indirect indexed: lda ($12,s),y
• Absolute indirect: lda ($1234)
• Absolute indirect long: lda [$1234]
• Absolute indexed indirect: lda ($1234,x)
• MVN/MVP: mvn $7E,$7F

The pseudo ops .as, .al, .xs & .xl are used to decide what size accumulator and index mode for the assembler to use. .as and .xs are for 8bit operands, and .al and .xl are for 16 bit operands.

I've found it handy to define these macroes:

#define SET_8_BIT_A()   sep #$20 : .as
#define SET_16_BIT_A()  rep #$20 : .al
#define SET_8_BIT_X()   sep #$10 : .xs
#define SET_16_BIT_X()  rep #$10 : .xl

#define SET_8_BIT_AX()  sep #$30 : .xs : .as
#define SET_16_BIT_AX() rep #$30 : .xl : .al

In addition to these modes, there are several operand prefixes that can be used to force a certain operand size/type.

• lda !$f0 would use 16-bit address instead of direct page.
• lda @$1234 would use a long (24-bit) address instead of absolute 16-bit address.
• lda #<var can be used to load the lower 8-bits of an external variable.
• lda #>var can be used to load the upper 8-bits of an external variable.
• lda #^var can be used to load the segment part (8-bits) of an external variable.
• lda @var,x can be used to use absolute-indexed-long mode instead of the default, absolute.

Expressions are supported. These are valid code:

• bra somewhere+1
• lda #!address + $100
• ldy #$1234 + ($6C * 3)

        ; Space-fill the buffer to end
        phx
         cpx #$0010
         bcs +     ;jumps to the next "+"
         SET_8_BIT_A()
         lda #$FF
-        sta $94A0,x
         sta $94B0,x
         inx
         cpx #$0010
         bcc -     ;jumps to the previous "-"
         SET_16_BIT_A()
+        lda W_VRAMADDR
         sta @$002116
        pla