Commodore CBM - POKE to the Screen; Calculating the Screen Address

POKE TO THE SCREEN
The programming of individual characters anywhere on the screen is most
effectively done
with the POKE statement. POKE allows a eharacter to be
placed anywhere
on the sereen by POKEing its eharaeter value into a sereen
location
in memory, whieh POKEs it onto the sereen.
Memory
Screen
Poke a
'1'
in Location X
xl+
1
'1' ...
on the Screen
\
location X
The PET sereen is Iike a gr id of 1000 squares; 40 columns by 25 rows: .
columns
rows
961 1000
One character may be displayed in each square. Ali 1000 sereen locations are
assigned
an address and spaee in memory. Memory sereen spaee begins at
address 32768 for square 1 (row 1, eolumn 1) and ends at address 33767 for
square
1000 (row 25, eolumn 40). Memory address 32768 is screen location
(1.1). address 32769 is screen location (1.2). etc. Figu re 5-12 shows the correla-
tion between
PET screen locations and their corresponding memory space and
addresses.
CALCULATING THE SCREEN ADDRESS
To find the screen address in memory for each screen location. manual
counting of each square
is unnecessary. The following equation is a simpler and
more efficient method to calculate the memory address:
Memory Address of Screen Location = 32768+(column-l)+(40 (row-))
Enter the column and row numbers of any screen position into the equation to
find its memory address. To demonstrate. enter the values 5 and 3 to find the
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