.\" $Header$
.TH PDP_AS 6 "$Revision$"
.ad
.SH NAME
pdp_as \- assembler for PDP 11
.SH SYNOPSIS
~em/lib.bin/pdp/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 pdp11 has seven general registers, numbered r0 through r7. 
Of these, r6 is the stack pointer and can also be referenced to by \fIsp\fP,
r7 is the program counter and has \fIpc\fP as synonym. There are also six
floating-point registers fr0 through fr5, but the names r0 through r5 can
also be used. From the context will be derived what kind of register is meant.
.IP "addressing modes"
.nf
.ta 8n 16n 24n 32n 40n 48n
syntax		meaning (name)

reg		contents of register reg is operand.
		(register)

(reg)		contents of reg is address of operand.
		(register deferred)

(reg)+		as (reg), but after the operand is fetched
		the contents of reg is incremented by the
		size of the operand. (auto-increment)

*(reg)+		contents of reg points to address of the operand.
		after the operand is fetched, reg is incremented
		by two. (auto-increment deferred)

-(reg)		as (reg), but before the operand is fetched
		the contents of reg is decremented by the
		size of the operand. (auto-decrement)

*-(reg)		before the operand is fetched, reg is decremented
		by two. then the contents of reg points to the
		address of the operand. (auto-decrement deferred)

expr(reg)	value of expr + contents of reg yields address
		of operand. (index)

*expr(reg)	value of expr + contents of reg yields pointer
		to address of operand. (index deferred)

$expr		the value of expr is the operand. (immediate)

*$expr		the value of expr is the address of the operand.
		(absolute)

expr		expr is address of operand. (relative)

*expr		expr points to the address of the operand.
		(relative deferred)

.fi
.IP "condition code instructions"
Two or more of the "clear" instructions (clc, cln, clv, clz), or
two or more of the "set" instructions (sec, sen, sev, sez) may be
or-ed together with `|' to yield a instruction that clears or sets two or more
of the condition-bits. Scc and ccc are not predefined.
.IP "extended branches"
The assembler recognizes conditional branches with a "j" substituted for
the "b". When the target is too remote for a simple branch, a converse branch
over a jmp to the target is generated. Likewise jbr assembles into either br
or jmp.
.IP "floating-point instructions"
The names of several floating-point instructions differ from the names
in the handbook mentioned below. Synonyms ending in "d" for instructions ending
in "f" are not recognized. Some instructions have different names; the mapping
is as below.
.nf
.ta 8n 16n 24n 32n 40n 48n

handbook		pdp_as

ldcif, ldclf,
ldcid, ldcld		movif

stcfi, stcfl,
stcdi, stcdl		movfi

ldcdf, ldcfd		movof

stcdf, stcfd		movfo

ldexp			movie

stexp			movei

ldd, ldf		movf

std, stf		movf

.fi
The movf instruction assembles into stf, when the first operand is one of the
first three floating-point registers, otherwise it assembles into ldf.
.IP sys
This instruction is synonymous with trap.
.SH EXAMPLE
An example of pdp11 assembly code.
.nf
.ta 8n 16n 24n 32n 40n 48n

!this is the routine that reads numbers into r0
!the number is terminated by any non digit
!the non digit is left in r1
.sect .text
innum:	clr r3		!r3 will accumulate the number
inloop:	jsr pc,_getchar	!read a character into r0
	cmp r0,$0121	!is it a Q?
	jeq quit
	cmp r0,$48	!is the character a digit? 
	jlt indone	!digits 0-9 have codes 060-071 octal
	cmp r0,$56
	jgt indone
	mul $10,r3	!r3 = 10 * r3
	sub $48,r3	!convert ascii code to numerical value
	add r0,r3	!r3 = old sum * 10 + new digi
	jbr inloop

indone:	mov r0,r1	!put the first non digit into r1
	mov r3,r0	!put the number read into r0
	rts pc		!return to caller

.fi
.SH "SEE ALSO"
uni_ass(6),
ack(1),
ack.out(5),
.br
PDP11/60 processor handbook, Digital Equipment Corporation, 1977
.SH BUGS
You cannot use *reg in place of (reg). Likewise *(reg) is not understood as
*0(reg).
.PP
Expressions are computed in two bytes, even the ones in .data4 lists.