Tynemouth MINI PET 40/80, Technical Manual, Page: 21 / 28

Tynemouth MINI PET 40/80
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VIDEO
The video circuitry of the Mini PET is very different to that of an original PET, but implements the same
functionality as a 2001 or closer to a 2001N PET. 1K-2K of video RAM is used to store a screen made up of one
byte for each of 40 or 80 x 25 characters. The highest bit of each byte controls whether the character is
inverted. The remaining seven bits of this byte form the index into the character ROM. This is made up of 128
8 x 8 character bitmaps. These include graphical characters, but there is no pixel graphics control.
The video RAM used on the Mini PET is dual port RAM, meaning that the CPU and the video circuitry can
access the video RAM without any conflicts. To draw a character on the screen, the PET writes it to the
appropriate location in the video RAM. The video circuitry continually reads the RAM and draws a screen. The
two are therefore isolated and operate separately. The video circuit on an original PET will show a screen full
of random characters even if the CPU is removed (or there is some fault which prevents it running code that
will write to the video RAM). The Mini PET exhibits the same isolation.
The PET character ROM was 2K in size, 8 x 8 bits of pixel data for two character sets of 128 characters each
(128x2x8 = 2048). The PET treats this as two sets of 256 characters, the second 128 characters being the
inverse of the first 128. In the PET (and the original Mini PET), this was achieved using a separate latch for
character address D7 which inverted the output of the shift register. To simplify things (and because ROM is a
lot cheaper now than it was in 1978), the ROM has been doubled in size and the inverse characters burned
into the ROM. This removed the need for the latch and further output synchronisation.
The smallest size of parallel EPROM currently available is 32K, so even with the extra inverted duplicates, there
is still space for multiple 4K character sets. On the Mini PET, four sets are provided, with DIP switches to select
which set to use.