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.pa 1
.he '3/15/72''FPTRAP (III)'
.ti 0
NAME		fptrap  --  PDP-11/45 floating point simulator
.ti 0
SYNOPSIS	.globl fptrap
sys ilgins; fptrap
.ti 0
DESCRIPTION	fptrap______
is a package which picks up instructions which are illegal
for the PDP-11/20, and if they correspond to 11/45 floating
point instructions, simulates their operation.
The following instructions are supported:

.	ldfps	src		(not in assembler)
.	stfps	dst		(not in assembler)
	clrf	fdst
	tstf	fsrc
	absf	fdst
	negf	fdst
	mulf	fsrc,fr
	modf	fsrc,fr
	addf	fsrc,fr
	movf	fsrc,fr	(=ldf)
	movf	fr,fdst	(=stf)
	subf	fsrc,fr
	cmpf	fsrc,fr
	divf	fsrc,fr
.	movei	fr,dst	(=stexp)  (not in assembler)
.	movie	src,fr	(=ldexp)  (not in assembler)
	movfi	fr,dst	(=stcfi)
	movif	src,fr	(=ldcif)
	movfo	fr,fdst	(=stcxy)
	movof	fsrc,fr	(=ldcyx)

Here src___ and dst___ stand for source and destination, fsrc____
and fdst____ for floating source and destination, and fr__ for
floating register.
Notice that the names of several of the opcodes have changed.
The only strange instruction
is movf____, which turns into stf___ if its source
operand is a floating register, and into ldf___ if not.
The simulator sets the floating condition codes
on both ldf___ and stf___.
The 11/45 hardware does not set
the fcc on stf.

Short and long format for both floating point numbers
and integers is supported.  Truncation mode is always in effect.
Traps for overflow and other arithmetic errors are not supported.
Illegal instructions or addresses cause a simulated trap
so that a core image is produced.

The condition code bits are maintained correctly.

For floating-point source operands, immediate mode ((pc)+) is
not supported, since the PDP-11/45
handbook is not clear on what to do about it.

After an arithmetic error the result is generally

The arithmetic is always done in double-precision, so exact
but unrounded results are to be expected in single-precision
mode.  Double precision results are probably less correct
than the hardware will be.

The lower parts of the floating registers become meaningless
during single-precision operations.
.ti 0
FILES		kept in /usr/lib/liba.a
.ti 0
SEE ALSO	PDP-11/45 handbook, ilgins(II)
.ti 0
DIAGNOSTICS	trap, c-bit, v-bit
.ti 0
BUGS		see above