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man/6809_as.1
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man/6809_as.1
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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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reg The operand of the instruction is in `reg'.
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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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<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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>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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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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#expr The value of `expr' is one- or two-byte immediate
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data. (immediate)
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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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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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, 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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(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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(, 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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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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(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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,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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(,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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,-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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(,--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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.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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compgo: lda #contby
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ldx #table - 2 !start of table
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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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