Low/high order byte of v9938 vram address

By kanageddaamen

Rookie (25)

kanageddaamen's picture

04-10-2017, 01:58

Hey all

For the application I am developing with the v9938 as a vdp, I need a portion of the vdp's memory space mapped to sram and a portion mapped to rom. I already know I need to latch the 8 address bits on \ras,.

What I am wondering is if anyone knows if the 8 address bits during \ras are the lower or uppper 8 bits of the memory address

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By PingPong

Prophet (3448)

PingPong's picture

04-10-2017, 18:58

the v9938 have a strange method of addressing ram. did you know that it use the paged/burst mode read?
effectively you have three CAS lines on v9938 instead of one..... look @the signals

CAS0
CAS1
CASX

the way the final user sees the vram (as a linear space of 128KB) is somewhat different from the physical addressing.
pratically vdp alternates between CAS0 and CAS 1 when doing linear addressing so i think that you want to know more about this strange addressing mode.
CAS X is for the so-called-non-displayable block of 64KB, the expanded ram

By kanageddaamen

Rookie (25)

kanageddaamen's picture

04-10-2017, 22:04

I was originally intending to use the CAS1 signal as bit 16 of the final 128k address space but it sounds like what you are saying is I should use CAS0 as bit 0.

I do need to still know if the 8 address signals output during \ras are the upper or lower portion (or some other strange arrangement) of the 64k linear address for each CAS signal so I can properly generate my chip select signals for the sram and rom

Is there any documentation on the v9938 physical addressing?

By PingPong

Prophet (3448)

PingPong's picture

06-10-2017, 08:25

http://map.grauw.nl/articles/vdp-vram-timing/vdp-timing.html
v9938 data/address bus was analyzed with a logic analyzer.
This is primary focused to the vram access timing, but does touch at a decent level even what physically happens when vdp need to write or read data from/to vram for various needs.

Read this with physical addressing in mind, not with timing issue in mind that is the main reason of existence of the doc.

Hope this helps.

By Edevaldo

Master (131)

Edevaldo's picture

07-10-2017, 06:55

RAS is a page number and CAS is the address within the page.