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+.\" $Header$
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+.TH 6809_AS 1
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+.ad
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+.SH NAME
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+6809_as \- assembler for 6809
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+.SH SYNOPSIS
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+/usr/em/lib/6809_as [options] argument ...
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+.SH DESCRIPTION
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+This assembler is made with the general framework
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+described in \fIuni_ass\fP(6).
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+.SH SYNTAX
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+.IP registers
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+The 6809 contains four 8-bit registers registers:
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+two accumulators (a and b),
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+a direct page register (dp),
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+and a condition code register (cc),
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+and five 16-bit registers:
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+two index registers (x and y),
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+a user an a hardware stack pointer (u resp. s),
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+and a program counter (pc).
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+The index registers and the stack pointers are indexable.
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+Accumulators a and b can be concatenated to form
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+the double accumulator d,
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+of which a is the high and b is the low byte.
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+An instruction that refers to accumulator a
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+has an "a" as last character.
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+In the same way a "b" means that the instruction
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+uses b as accumulator.
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+.IP "pseudo instructions"
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+The 6809 assembler recognizes one additional instruction
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+that is not translated into a machine instruction: setdp.
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+It expects an expression as argument.
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+This is used for efficient address encoding of some addressing
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+mode (see below).
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+.IP "addressing modes"
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+.nf
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+.ta 8 16 24 32 40 48
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+syntax meaning (name)
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+
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+reg The operand of the instruction is in `reg'.
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+
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+reglist `reglist' is a either list of registers, seperated
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+ by ','s, or the word "all". It encodes in a register
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+ save mask, where "all" means all registers, that can
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+ be used by the push-pull instructions pshs, pshu,
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+ puls, and pulu.
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+
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+<expr The one-byte value of `expr' is an address within
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+ a 256-byte page. The particular page in use is
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+ indicated by the contents of dp, so `expr' is the
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+ low byte of the effective address of the operand,
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+ and dp the high byte. (direct)
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+
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+>expr The two-byte value of `expr' is the exact memory
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+ address. Not that this mode always requires one
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+ byte more than "<expr". (extended)
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+
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+expr The value of `expr' is an address.
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+ Except for long branches, this value is examined
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+ first to see if a short encoding is possible.
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+ When the instruction is a short branch, the value
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+ is checked to see if the address is not too remote,
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+ because in that case this branch is automatically
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+ replaced by a long branch. When the instruction is
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+ not a branch, the high byte of the value is compared
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+ with the value of the argument of the last setdp
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+ pseudo. If they are equal, this mode is replaced by
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+ "<expr", else by ">expr".
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+ (relative for branch-instructions)
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+
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+#expr The value of `expr' is one- or two-byte immediate
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+ data. (immediate)
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+
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+(expr) The value of `expr' is a pointer to the address
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+ of the operand. (indirect)
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+
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+expr, reg The value of `expr' added to the contents of `reg'
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+ (which must be a 16-bit register) yields the
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+ effective address of the operand.
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+ (constant-offset indexed)
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+
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+, ireg The contents of `ireg' (which must be indexable)
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+ yields the effective address of the operand.
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+ (constant-offset indexed)
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+
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+(expr, reg) The value of `expr' added to the contents of `reg'
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+ (which must be a 16-bit register) yields a pointer
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+ to the effective address of the operand.
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+ (constant-offset indexed indirect)
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+
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+(, ireg) The contents of `ireg' (which must be indexable)
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+ yields a pointer to the effective address of the
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+ operand. (constant-offset indexed indirect)
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+
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+ac, ireg The contents of `ac' (which must be an accumulator)
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+ added to the contents of `ireg' (which must be
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+ indexable) yields the effective address of the
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+ operand. (accumulator indexed)
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+
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+(ac, ireg) The contents of `ac' (which must be an accumulator)
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+ added to the contents of `ireg' (which must be
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+ indexable) yields a pointer to the effective address
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+ of the operand. (accumulator indexed indirect)
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+
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+,ireg+
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+,ireg++ The contents of `ireg' (which must be indexable) is
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+ used as effective address of the operand. After that
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+ it is incremented by 1 (+) or 2 (++).
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+ (auto-increment)
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+
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+(,ireg++) The contents of `ireg' (which must be indexable) is
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+ used as a pointer to the effective address of the
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+ operand. After that it is incremented by 2.
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+ (auto-increment indirect)
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+
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+,-ireg
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+,--ireg `ireg' (which must be indexable) is decremented
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+ by 1 (-) or 2 (--). After that, its contents is used
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+ as effective address of the operand.
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+ (auto-decrement)
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+
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+(,--ireg) `ireg (which must be indexable) is decremented by 2.
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+ After that, its contents is used as a pointer to the
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+ effective address of the operand.
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+ (auto-decrement indirect)
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+
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+.fi
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+.SH "SEE ALSO"
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+uni_ass(6),
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+ack(1),
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+.br
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+MC6809 preliminary programming manual, Motorola Inc., First Edition, 1979
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+.SH EXAMPLE
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+An example of 6809 assembly code.
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+.nf
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+.ta 8 16 24 32 40 48
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+ contby = 80
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+
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+ compgo: lda #contby
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+ ldx #table - 2 !start of table
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+
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+ clrb
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+ co1: addb #2
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+ lsra
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+ bcc co1
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+ jmp (b, x) !accumulator offset indirect
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+.fi
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