Your Commodore - Commodore Is Awesome

I
n data can be input into your code by
t
means of the ROM routines.
h
The trivial method of inputing data
is to POKE values into the relevant
i
locations used by the code. This
s method is good enough and may be
amandatory
if you're using a Basic
rdriver
program and you want to use a
tcompiler.
Those of you with more
i
refined taste will want something
better.
c
One of the most important routines
lin
the Basic ROM starts at $AD9F•
eThis
evaluates input expressions and
, checks the syntax. There are a large
I number of entry points to this routine
w
which can be used for our benefit.
a
n
Getting into M/C
t
tThere
are two commands for the
o accessing of machine code from Basic.
cFirst
we have the USR command. It's
ofuill
syntax is:
n
s
Variable — USR(expression)
i
The expression within the brackets is
devaluated
and stored in FAC/1
e(floating
point accumulator no I). This
r value is then used in your code. The
hentry
point of your code must fi
orst
stored bin ethe
vector in locations $0310
to $0312. On leaving the routine, the
wvalue
in FAC/1 is transferred to the
variable in the calling instruction. Since
you can only pass one parameter, this
instruction is of limited value.
The more frequently used
command is SYS. This simply calls the
machine code starting at the specified
MACHINE CODE
Welcome to
the Machine
ROM routines revealed. Read on to find out how to
input data into your machine code
By Allen Webb
address. Since SYS doesn't support
parameters, we must use some coding
to add them. The most usual syntax
adopted is of the form:
SYS address, parameter 1, parameter
2,.. „etc
After your code is entered, the
Basic text pointer is pointing at the
character after the SYS. We can check
this character using four routines:
I LISTING 2
40 CHKCOMMA $AEFD
50 CHKLPAR = $AEFA
60 CHKRPAR = $AEF7
70 EVALTERM $AD8A
80 FLPTOINT = $B7F7
85 CHKCHAR = $AEFF
90 PARAM1 = $AA
95 PARAM2 = $F8
100 JSR CHKCOMMA IS 1ST COMMA THERE ?
110 JSR CHKLPAR OPEN BRACKET ? •
120 JSR EVALTERM ; YES — GET VALUE
130 JSR FLPTOINT
140 LDA $14 ; AND SAVE IT IN
150 STA PARAM1 RESULT 1
160 LDA $15
170 STA PARAM11-1
180 JSR CHKRPAR CLOSING BRACKET ?
190 JSR CHKCOMMA NEXT COMMA ?
200 JSR EVALTERM YES — GET VALUE
205 JSR FLPTOINT
210 LDA $14 ; SAVE IT IN
220 STA PARAM2 RESULT2
230 LDA $15
240 STA PARAM24-1
250 RTS
YOUR COMM(MORE november 1986:85
$AEF7 — check that character is a right
bracket
SAEFA — check that character is a left
bracket
SAUD — check that character is a
comma
$AEFF check that character is that
held in the accumulator
These routines give a SYNTAX
ERROR if the character is not that
sought. If the character is correct, the
1