# 6502 assembler

6502 is one of the simplest CPUs out there and its assembler is
also simple. We have 3 types of opcodes: inherent, addressed
and branches.

As with other assemblers, all ops described below have a ","
suffix. For example, you write "NOP," rather than "NOP"

# Inherent

Inherent opcodes are called without argument.

BRK NOP RTI RTS
CLC CLD CLI CLV
SEC SED SEI 
DEX DEY INX INY
PHA PLA PHP PLP
TAX TXA TAY TYA TSX TXS

# Addressed

Addressed opcodes take an address argument which needs to be
filtered through address mode words.

#    Immediate
<>   ZeroPage
<X+> ZeroPage+X
<Y+> ZeroPage+Y
()   Absolute
(X+) Absolute+X
(Y+) Absolute+Y
[X+] Indirect+X
[]Y+ Indirect+Y

The indirect notations are not a typo, they're to illustrate
the difference in indirection scheme between X and Y. See 6502
datasheet.

Example usage:

42 # LDA,
$fe <> LDX,
$1234 () STY,

Not all address modes are legal with all ops below. This
assembler is not going to tell you when your combo is illegal,
it's just going to spit invalid code. The op list below indicate
valid address modes for each op.

We have a special situation with ASL/LSR/ROL/ROR: they can
target the accumulator. We have no addressing mode for this.
Instead, we have a special "inherent" op (no argument) for these
4 cases: ASLA/LSRA/ROLA/RORA. The "A" in the list below indicate
that.

ADC # <> <X+> () (X+) (Y+) [X+] []Y+
SBC # <> <X+> () (X+) (Y+) [X+] []Y+
CMP # <> <X+> () (X+) (Y+) [X+] []Y+
CPX # <> ()
CPY # <> ()
AND # <> <X+> () (X+) (Y+) [X+] []Y+
ORA # <> <X+> () (X+) (Y+) [X+] []Y+
EOR # <> <X+> () (X+) (Y+) [X+] []Y+
BIT <> ()
ASL A <> <X+> () (X+)
LSR A <> <X+> () (X+)
ROL A <> <X+> () (X+)
ROR A <> <X+> () (X+)
DEC <> <X+> () (X+)
INC <> <X+> () (X+)
LDA # <> <X+> () (X+) (Y+) [X+] []Y+
LDX # <> <X+> () (Y+)
LDY # <> <X+> () (X+)
STA # <> <X+> () (X+) (Y+) [X+] []Y+
STX <> <X+> () 
STY <> <X+> ()

# Branches

Conditional branches are all relative, unconditional branches
are absolute.

There are 2 absolute branching ops: JMP and JSR. They are called
with a single numerical argument. The indirect mode of JMP is
called through the special JMP[] op. Examples:

$1234 JMP,
$1234 JMP[],
$1234 JSR,

Relative branch words are called with a single byte argument
and are compatible with regular flow words:

$fe BEQ,
CLC, BEGIN, NOP, BR BCC,

An important limitation with 6502 is that there is no relative
unconditional branch word! This has important implications with
our regular flow words because it means that "JRi," for 6502
has to be hackish and spit out an absolute JMP. This works with
BR, BUT NOT FOR FMARK.

Therefore, in 6502 code, FMARK is broken with unconditional
jumps and can't be used. Conditional is fine though, so
IF,..THEN, works.

Relative jump words:

BCC BCS (C=0/1)
BNE BEQ (Z=0/1)
BPL BMI (N=0/1)
BVC BVS (V=0/1)