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Simple 6502 assembler

License: MIT License

Assembly 0.44% Go 96.09% Shell 2.52% Makefile 0.95%

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a2asm's Issues

Bad output when calling JMP to a label that's yet to be defined

	ORG $800
	JSR MAIN
	JMP EXIT
EXIT	EQU $3D0
INIT	EQU $FB2F
HOME	EQU $FC58
MAIN	JSR INIT
	JSR HOME
	RTS

Should assemble to:

20 06 08 4C D0 03 20 2F
FB 20 58 FC 60

20 06 08 4C 06 08 20 2F
FB 20 58 FC 60

Note: JMP EXIT becomes JMP $806 instead of JMP $3D0. Confirmed that MERLIN produces the expected result.

Add support for literals in arithmetic

XPOS	EQU 1
YPOS	EQU 0
	LDA SHIPS+XPOS,X
	RTS
SHIPS	DFB 00,00

Should assemble to BD 04 80 60 00 00, but fails with error:

expected hex, binary, or decimal literal; got XPOS

Fix high-byte subtraction

PRPLA EQU $FDE2
      LDX #>PRPLA-1

Should assemble to A2 FD, not A2 FE.

I think this is happening because num is unsigned, so -1 becomes 0xFFFF (then saved as 0xFF).

Adding 0xFF to 0xFDE2 results in 0xFEE1 and the high-byte 0xFE is returned.

Repro

func TestHighByteSubtraction(t *testing.T) {
	out := bytes.NewBuffer(nil)
	prg := strings.NewReader(`
PRPLA	EQU $FDE2
	LDX #>PRPLA-1
	`)

	_, err := Assemble(out, prg, true)
	if err != nil {
		t.Error(err)
		return
	}

	// 0000-	A2 FD   	LDX #>PRPLA-1
	expected := []byte("\xA2\xFD")

	actual := out.Bytes()
	if !bytes.Equal(expected, actual) {
		t.Errorf("Expected %v; got %v", expected, actual)
	}
}