a E i - Commodore Is Awesome

Memory Allocation
The following addresses are all in
hexadecimal, the descriptions explain
what happens in that area of memory.
055F - 059F - function keys (reassigned).
4000 - 4104 - text dump program.
4105- 4283 - graphic dump program.
4284 - 4290 - save page zero in safe
area routine.
4291 - 4291) - return safe area to page
zero routine.
429E - 442D - graphic screen address
table,
442E - 4434 - unprocessed graphic
character.
4435 - 443C processed graphic
character.
443D - 457E - output buffer (graphics
& text).
457F- 467E - safe area for temp store
of page zero.
Text Dump
The text dump is basically a code converter.
Firstly, all of page zero is stored
in a safe area at $4571
7 $4284. by This t hwill e protect all vectors set
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The contents of
location
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$FF13 are then checked to see
if the screen in displaying upper or
lower case text. A channel is then
opened to the printer. The appropriate
font will be selected according to the
contents of $FF13 (ref: pages 40-41 of
the MPS-801 Users/Manual). Locations
$22 and $23 are loaded with the
start address of the text screen
($0000). The X-register (row counter)
and the Y-register (column counter on
screen) are both set to zero. Location
$4B is a counter which keeps track of
how many characters are to be sent to
the printer to produce a row of print.
The value stored in $413 will vary
depending on how many reversed
characters are to be sent. Every time a
reversed character is to be printed,
three characters have to be sent: RVS
ON, the character, R VS OFF.
Location $4B is set to zero for each
row being processed, as is the Yregister.
A problem exists with the text
dump, which is the reason for the
length of the text dump program. The
characters stored in text screen
memory are stored as screen display
code. Characters to be sent to the
printer must be sent as ASCII code
(ref: pages 192-196 of the Plus/4 User
PLUS/4 AND MPS 80111
0
6
Byte number.
11
1 2 3 4 5 6 7 8 9 0 1
This shows the pattern of bytes and bits as they
appear on the printer. (First row represented.)
Manual). As can be seen from the two
tables in the User Manual, each
character has two different values.
Conveniently the characters have been
grouped into sets of 32. It is the order
of the sets that is different. The
program takes a character from the
text screen memory, and finds out
which set of 32 it is in (screen display
code).
From this point it is merely a case
of adding a pre-defined number to do
the conversion. The method used to
find which set a character is in. is
purely a case of subtraction. The
character value is taken, #$20 subtracted
and accumulator is checked, to see
if it contains a positive number. If it
does contain a positive number then
the character must belong to a higher
group of 32 characters. In this case the
MO is added back to the number and
the analysis is continued in the same
way by substracting # S40, # $60 and
/480 in that order. Each time, if the
answer is positive, the number
subtracted is replaced. When the result
in the accumulator is not positive then
the program has located which block
of 32 the character Ilelongs to. The
program will then add # $60, #140,
#SCO depending whether the character
is in the first, second, third or fourth
set of 32 characters respectively.
Example
Assume the character to be processed
is the spade symbol. As can he seen in
the SDC table it has a screen display
YOUR COMMODORE january 1987
104
Figure 2.
,• • • • • • • • • • =m• 1• R E =1 emmum • • • • • • • •
33
12
90
code of 65 (decimal) which is $41 in
hex. To convert to ASCII the program
follows this method:
41$41 screen display code for spade
symbol
#82I substract # $20
ANSWER IS POSITIVE SO NOT IN
THIS GROUP OF 32.
#$41 replace #$20
#$01 substract # $40
ANSWER IS POSITIVE SO NOT IN
THIS GROUP OF 32_
tt84 replace# $40
#$E1 substract #$60
NOTE: THE NUMBER HAS
WRAPPED AROUND THE NEG-
ATIVE FLAG IN THE PROCESSOR
HAS BEEN SET, THIS IS THE
CORRECT GROUP OF 32
CHARACTERS.
#$61 add pre-defined number #S80.
THIS IS THE ANSWER !!!#$61 IS 97
IN DECIMAL. CHR$(97) IS THE
SPADE SYMBOL.
However, just before the above
process is executed, the screen display
code currently being processed is
ANDed with #$80. If the result is less
than, or equal to,#87F then the above
process is executed in the normal way.
If however the result is greater than
#$80 then the program realises it is
dealing with a reversed character, so it
stores# $12 in the next free location in
the output buffer ($443D plus the
contents of $48). This sets the printer
up for RVS ON. The value in $48 is
incremented by #$01. The character is
then converted in the normal way and