# 6809 assembler specificities

Load with "6809A".

First, the 6809 stands out by being big-endian. It doesn't
change much in terms of assembler usage, but it's a good idea
to keep it in mind.

Then, it stands out by having few "targetable" registers. It
onlt has A, B and D accumulators and X, Y, U and S registers are
targeted directly by only a handful of operations. Therefore,
6809 assembly language designer decided to decline every ops
with all their possible targets. For example, the "ADD" op
has 3 forms: ADDA, ADDB and ADDD. This assembler follow this
design and has an op word for every form.

Then, it stands out by having a vast array of addressing modes.
This significantly impact usage: Except for inherent operations
(ops that don't require any argument), all arguments passed to
operations have to first pass through an "adressing word". For
example, "<>" means the "Direct addressing". Example usage:

$42 <> CMPA,

This line is equivalent to "cmpa $42" in "regular assembly".
Addressing words are:

* "#" --> Immediate
* "()" --> Extended addressing
* "[]" --> Indirect Extended
* Indexed:
  * "R+N" --> Constant Offset indexed
  * "R+0" --> Shortcut for "0 R+N"
  * "R+R" --> Accumulator Offset indexed
  * "R+", "R++", "-R", "--R" --> Auto-increment indexed
  * All index words have their indirect forms: "[R+N]", "[R++]",
    etc..

Index words above are declined and R is a placeholder. Actual
words have actual registers, for example, "X+N", "Y+D", "[S+]",
etc. Example full usages:

42 # CMPB,
L1 @ () LDA,
X+A ADDB,
[Y++] ADCA,

# The case of PSH, PUL, TFR, EXG

TFR and EXG are exceptions to the above rule that all arguments
go through an adressing word. The 6809 define register constants
for usage with TFR and EXG and can be used directly. Example:

A B TFR, ( copy A into B )
U S EXG, ( exchange U and S )

PSH and PUL are even bigger exceptions. Their argument *follow*
the op mnemonics and this argument is a list of single letter
registers: $ (for PC), S, U, Y, X, % (for DPR), A, B, D
C (for CCR), @ for all. Order doesn't matter. S/U mean the same
thing. D means A and B. Examples:

PSHS, ABUXY
PULU, $
PSHU, @

# Branching

The 6809 assembler supports regular branching words but has
special provisions for 16-bit relative branching, something that
not all arches support.

The "L" versions of relative branches are present, but because
flow words only support 8-bit branching, it's not of much use.

# Instructions

Next to each operation, in [] brackets, are supported addressing
modes:
M = Immediate D = Direct I = Indexed E = Extended H = Inherent

When forms have the same signature, they are grouped in ()
brackets.

ABX          [H]
ADC(A,B)     [MDIE]
ADD(ABD)     [MDIE]
AND(AB)      [MDIE]   ANDCC [M]
ASL(AB)      [H]      ASL   [DIE]
ASR(AB)      [H]      ASR   [DIE]
BIT(AB)      [MDIE]
CLR(AB)      [H]      CLR   [DIE]
CMP(ABDXYUS) [MDIE]
COM(AB)      [H]      COM   [DIE]
CWAI         [M]
DAA          [H]
DEC(AB)      [H]      DEC   [DIE]
EOR(AB)      [MDIE]
EXG          SPECIAL
INC(AB)      [H]      INC   [DIE]
JMP          [DIE]
JSR          [DIE]
LD(ABDXYUS)  [MDIE]
LEA(XYUS)    [I]
LSL(AB)      [H]      LSL   [DIE]
LSR(AB)      [H]      LSR   [DIE]
MUL          [H]
NEG(AB)      [H]      NEG   [DIE]
NOP          [H]
OR(AB)       [MDIE]   ORCC  [M]
PSH(US)      SPECIAL
PUL(US)      SPECIAL
ROL(AB)      [H]      ROL   [DIE]
ROR(AB)      [H]      ROR   [DIE]
RTI          [H]
RTS          [H]
SBC(AB)      [MDIE]
SEX          [H]
ST(ABDXYUS)  [DIE]
SUB(ABD)     [MDIE]
SWI          [H]
SWI2         [H]
SWI3         [H]
SYNC         [H]
TFR          SPECIAL
TST(AB)      [H]      TST   [DIE]

Branches: All words below have a "L" form for a 2b displacement.
Example: BRA --> LBRA

BCC BCS BEQ BGE BGT BHI BHS BLE BLO BLS BLT BMI BNE BPL BRA BRN
BSR BVC BVS