8bit ISA on 16bit bus \ VOGONS

8bit ISA on 16bit bus

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First post, by vladstamate

Posted on 2017-02-12, 15:11

vladstamate Offline

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Rank: Oldbie
Posts: 967
Joined: 2015-08-23, 01:43

Hi all,

Say I have an 8bit EGA card in a 16bit system (like a 286). If I write 16bit value to EGA memory ( something like )

MOV AX, B800h
MOV DS, AX
MOV WORD PTR [0], 20h

Who is responsible for breaking the 16bit write into 2 8bit writes? The CPU is agnostic of the 8 vs16bit ISA card right? There must be some glue TTL on the motherboard that understands that right?

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Reply 1 of 1, by Jepael

Posted on 2017-02-12, 19:36

Jepael Offline

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Joined: 2005-06-15, 19:28
Location: Finland

Yes, the card will pull IOCS16 or MEMCS16 low as an indication when 16-bit access is supported. If it does not, then the motherboard circuitry converts the 16-bit access into two 8-bit accesses.

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