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External and internal data width

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jeffythedragonslayer

Post subject: External and internal data width

Posted: Fri Oct 29, 2021 8:16 am

Joined: Sun Oct 03, 2021 2:17 am
Posts: 114

The Wikipedia page about the 65816 has an infobox containing this:

Quote:

Data width
8 (external)
16 (internal)

External and internal to what? Is it trying to say that memory is fetched from RAM in 8 bit chunks but registers inside the 65816 are 16 bit?

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drogon

Post subject: Re: External and internal data width

Posted: Fri Oct 29, 2021 8:30 am

Joined: Wed Feb 14, 2018 2:33 pm
Posts: 1399
Location: Scotland

jeffythedragonslayer wrote:

The Wikipedia page about the 65816 has an infobox containing this:

Quote:

Data width
8 (external)
16 (internal)

External and internal to what? Is it trying to say that memory is fetched from RAM in 8 bit chunks but registers inside the 65816 are 16 bit?

Essentially yes.

There are flags to run on/off the 8/16 bit access/transfer mechanisms, so it can all by 8-bit or all 16-bit or some 8 and some 16 but when reading or writing a 16-bit value from RAM, then 2 RAM accesses are needed.

-Gordon

_________________
--
Gordon Henderson.
See my Ruby 6502 and 65816 SBC projects here: https://projects.drogon.net/ruby/

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GARTHWILSON

Post subject: Re: External and internal data width

Posted: Fri Oct 29, 2021 8:33 am

Joined: Fri Aug 30, 2002 1:09 am
Posts: 8428
Location: Southern California

jeffythedragonslayer, yes; in native mode, you can set the accumulator to 8 or 16 bits, and you can set the index registers to 8 or 16 bits. So if you have the accumulator set to 16 bits wide for example, and you do an LDA foobar, it will put the byte at foobar in the low 8 bits of the accumulator, and in the next cycle grab the content of foobar+1 and put that in the high 8 bits of the accumulator. INC foobar will do a 16-bit (two-byte) increment of the variable at foobar. See my article about the common misunderstandings surrounding, and the attractions of, the 65816.

_________________
http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?

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