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1024
RAM
Operating system and
BASIC working storage
8192
32768
33792
34816
35840
36864
49152
59392
61440
65536
RAM
User BASIC program
- - - - - - - - - - - - -
User Variables
Expansion RAM area-24K
RAM TV
1---------------
Images of TV RAM
Images of TV RAM
Images of TV RAM
Expansion ROM area·12K
ROM BASIC
1/0
ROM
operating system
FIgure 2.3. PET memory map
On every cycle of the TV screen (1/60 of a second), the hardware starts with the least address
($8000) in the screen memory and processes the screen data starting at the upper left-hand
corner of the screen. Each character in the memory is addressed into the character generator eight times,
giving us an
8
row high character on the screen. The character ROM that is used generates
8 dots each time it is addressed. These dots are serially fed to the screen, working from left to right and
top to bottom. This gives an 8 bit wide 8 bit tall character with no spaces between characters. The CRT
controller automatically changes the addressing of the character generating ROM, depending
on whether or not it is scanning the top line of a character, the second line of of a
character, etc.
There are two character sets stored in the ROM. You can change the character set on the
screen by POKEing memory address 59468 with a 14(a 12 turns it back) which turns it to
the second character set. After you have played with the screen a little bit, you may want
to try this feature to see if your PET performs this way. The second character set substitutes
lower case letters for the graphic set that is available in the first set.
To understand this, let us review how characters are represented in the PET and in the memory.
6
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