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80 col text trick
This was posted in comp.sys.mac.hardware. Anyone do something
similar?
In article <3ukh1e$6qh@ns.ccsn.edu>, Russell Mosemann <mose@ns.ccsn.edu> wrote:
> Heh, heh. This reminds me of the true story of a person who said
>he had found "the" resistor in the Apple II which, when cut, would change
>the monitor from 40 columns to 80 columns.
Back in 1979 I modified my Apple ][ clone to display 80 column text with
the addition of a few 74F series TTL chips, mostly combinational logic,
with no counters, and a 128 byte SRAM. No expansion slot was used.
The ]['s video output shift register can be clocked at 14.31818 MHz, giving
twice the video bandwidth going out. The question then becomes what to
do with the extra bandwidth, now that you've made every character on the
screen half their previous width.
What I did was to use the 128 byte SRAM to act as a row buffer. Since
the ]['s text and graphics memory addressing uses two pages, I switched
in the other page during the single blank line between rows of text to
load the row buffer. During the scan lines with "dots" on the screen, I
alternated between the actual data coming from the motherboard and the
stored data coming from the SRAM.
The next problem becomes how best to write the software driver to handle
this strange addressing scheme: alternate characters on the screen
reside in different video pages!
Anyway, at the time, I thought it was pretty cool. At least I hadn't
heard of anyone designing such a scheme at the time. If turned into a
product, the hardware cost would be far lower than the 80 column text
card that became big sellers at the time, and the software would have
been easier to handle also.
Michael M.Y. Hui (speaking privately) myhui@thlayli.newport-beach.ca.us
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