So
24
is
to
be
stored at location 134 (Iow byte). and 28
is
to
be stored at location
135 (high byte). The following instructions can be used:
10
AL=PEEK(134):AH=PEEK(135):
20
POKE
132.24:POKE
135.28:
REM
SAVE
CURRENT
POINTER
REM
TOP
OF
CORE
NOW
=
7192
1~1et
POKE
134
..
AL:
POKE
135
..
AH:
1113
Erm
REM
RE3TORE POINTER
Chapter
6:
USR
Since the accumulator
is
maintained in different system locations
on
the old
ROMs, the accumulator description
will
read
as
follows:
The parameter value
is
passed to the
USR
subroutine in system locations
that function
as
a floating point accumulator
(FAC)
for ail functions. The
FAC
resides
in
six bytes fram memory locations 176 to
181
(8°16-8516).
The
FAC
has
the following format:
O=positive
-1
=negative
(FAC
ating
nt)
178
179 180
181
Fraction
Flo
1
1
Poi
11
Sign
l
Exponent
f
Memory location: 1
76
177
Like floating point variables. the exponent
is
stored in excess 128 format. and the
fraction
is
normalized
with
the high-order bit
of
byte 177 (the high-order byte
of
the fraction) set to
1.
The difference between this format and the variable format
is
that the high-order 1
bit
is
present in byte 177 of the
FAC.
An extra byte
(181)
is
used to hold the sign
of
the fraction. (This
is
done for ease
of
manipulation by the
functions that use the
FAC).
1.
PET
User
Notes. Volume
1.
Issue
6.
Sept.-Oct. 1978.
pg.
14.
"Cassette File Usage Summary" by Jim Butterlield.
2.
Best
01
the
PET
GAZETTE.
pg.
38
"On
Data
Files:", by Michael Richter.
413
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