.\" $Header$
.TH 6809_AS 6 "$Revision$"
.ad
.SH NAME
6809_as \- assembler for 6809
.SH SYNOPSIS
~em/lib.bin/6809/as [options] argument ...
.SH DESCRIPTION
This assembler is made with the general framework
described in \fIuni_ass\fP(6). It is an assembler generating relocatable
object code in \fIack.out\fP(5) format.
.SH SYNTAX
.IP registers
The 6809 contains four 8-bit registers registers:
two accumulators (a and b),
a direct page register (dp),
and a condition code register (cc),
and five 16-bit registers:
two index registers (x and y),
a user an a hardware stack pointer (u resp. s),
and a program counter (pc).
The index registers and the stack pointers are indexable.
Accumulators a and b can be concatenated to form
the double accumulator d,
of which a is the high and b is the low byte.
An instruction that refers to accumulator a
has an "a" as last character.
In the same way a "b" means that the instruction
uses b as accumulator.
.IP "pseudo instructions"
The 6809 assembler recognizes one additional instruction
that is not translated into a machine instruction: setdp.
It expects an expression as argument.
This is used for efficient address encoding of some addressing
mode (see below).
.IP "addressing modes"
.nf
.ta 8n 16n 24n 32n 40n 48n
syntax		meaning (name)

reg		The operand of the instruction is in \fIreg\fP.

reglist		\fIreglist\fP is a either list of registers, seperated
		by ','s, or the word "all". It encodes in a register
		save mask, where "all" means all registers, that can
		be used by the push-pull instructions pshs, pshu,
		puls, and pulu.

<expr		The one-byte value of \fIexpr\fP is an address within
		a 256-byte page. The particular page in use is
		indicated by the contents of dp, so \fIexpr\fP is the
		low byte of the effective address of the operand,
		and dp the high byte. (direct)

>expr		The two-byte value of \fIexpr\fP is the exact memory
		address. Not that this mode always requires one
		byte more than "<expr". (extended)

expr		The value of \fIexpr\fP is an address.
		Except for long branches, this value is examined
		first to see if a short encoding is possible.
		When the instruction is a short branch, the value
		is checked to see if the address is not too remote,
		because in that case this branch is automatically
		replaced by a long branch. When the instruction is
		not a branch, the high byte of the value is compared
		with the value of the argument of the last setdp
		pseudo.  If they are equal, this mode is replaced by
		"<expr", else by ">expr".
		(relative for branch-instructions)

#expr		The value of \fIexpr\fP is one- or two-byte immediate
		data. (immediate)

(expr)		The value of \fIexpr\fP is a pointer to the address
		of the operand. (indirect)

expr, reg	The value of \fIexpr\fP added to the contents of \fIreg\fP
		(which must be a 16-bit register) yields the
		effective address of the operand.
		(constant-offset indexed)

, ireg		The contents of \fIireg\fP (which must be indexable)
		yields the effective address of the operand.
		(constant-offset indexed)

(expr, reg)	The value of \fIexpr\fP added to the contents of \fIreg\fP
		(which must be a 16-bit register) yields a pointer
		to the effective address of the operand.
		(constant-offset indexed indirect)

(, ireg)	The contents of \fIireg\fP (which must be indexable)
		yields a pointer to the effective address of the
		operand. (constant-offset indexed indirect)

ac, ireg	The contents of \fIac\fP (which must be an accumulator)
		added to the contents of \fIireg\fP (which must be
		indexable) yields the effective address of the
		operand. (accumulator indexed)

(ac, ireg)	The contents of \fIac\fP (which must be an accumulator)
		added to the contents of \fIireg\fP (which must be
		indexable) yields a pointer to the effective address
		of the operand. (accumulator indexed indirect)

,ireg+
,ireg++		The contents of \fIireg\fP (which must be indexable) is
		used as effective address of the operand. After that
		it is incremented by 1 (+) or 2 (++).
		(auto-increment)

(,ireg++)	The contents of \fIireg\fP (which must be indexable) is
		used as a pointer to the effective address of the
		operand. After that it is incremented by 2.
		(auto-increment indirect)

,-ireg
,--ireg		\fIireg\fP (which must be indexable) is decremented
		by 1 (-) or 2 (--). After that, its contents is used
		as effective address of the operand.
		(auto-decrement)

(,--ireg)	\fIireg\fP (which must be indexable) is decremented by 2.
		After that, its contents is used as a pointer to the
		effective address of the operand.
		(auto-decrement indirect)

.fi
.SH "SEE ALSO"
uni_ass(6),
ack(1),
ack.out(5),
.br
MC6809 preliminary programming manual, Motorola Inc., First Edition, 1979
.SH EXAMPLE
An example of 6809 assembly code.
.nf
.ta 8n 16n 24n 32n 40n 48n
	.sect .text
	contby = 80

	compgo:	lda	#contby
		ldx	#table - 2	!start of table

		clrb
	co1:	addb	#2
		lsra
		bcc	co1
		jmp	(b, x)		!accumulator offset indirect
.fi