External Memory Mapper

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Por Leo

Paragon (1236)

Imagen del Leo

02-08-2009, 20:07

it looks like an electrical problem like driver strength versus load problem , and this causing timing error
on some data adress line : D7 & A14
...
When you say it works with 8250 with 2mb internal ram how can you say that ?
if you do
print (inp(&hA8))
on the 8250 with 2mb , with and without mapper i guess you will have the same
value which means it does use biggest internal mapper ... well give a try ...
...

by the way , how can 2 mapper work together since they are sharing the same
ports FC FD FE FF ?

Por Erikie

Champion (324)

Imagen del Erikie

02-08-2009, 21:27

each mapper has its own ls670 that holds the values for those registers.
I use testmap to test the mapper functionality.
It tests with no errors on my expanded 8250 Smile

Por RetroTechie

Paragon (1563)

Imagen del RetroTechie

02-08-2009, 21:46

by the way , how can 2 mapper work together since they are sharing the same ports FC FD FE FF ?
Each mapper holds its own copy of these registers, but limited no. of bits (128K: 3 bits, 512K: 5 bits etc). For bits that 'apply' to each mapper, all mappers will return the same value -> no conflict.

Higher order bits are only controlled by the largest mapper (=no conflict), or not at all (=also no conflict). If not controlled, they usually read back as logic "1" (pull-up resistors on the databus), sometimes as "0". Ofcourse competent programmers won't read mapper registers anyway, since read back values are not guaranteed / trustworthy.

Por Erikie

Champion (324)

Imagen del Erikie

03-08-2009, 09:03

I tested the 2mb 8250 with and without mapper with port A8
it gives F0 without mapper and A0 with mapper Smile

Por Leo

Paragon (1236)

Imagen del Leo

03-08-2009, 10:20

ah ok , so it really use it ! i am surprised ...

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