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Vintage Computers Any vintage computer systems, calculators, video games etc., but with an emphasis on 1980s and earlier equipment. |
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22nd Nov 2020, 1:08 pm | #1 |
Octode
Join Date: Mar 2011
Location: North Yorkshire, UK.
Posts: 1,119
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Motorola 6800 & Mikbug question
A question for the 6800 experts thats been puzzling me for a while
I wonder why Mike Wiles put Mikbug at $E000-E1FF and not at the top of memory as required by the hardware for reset, nmi, swi etc. The document acknowledges that it relies on the reflected copy at $FE00-FFFF, which seems a weird and wasteful decision, actually imposing partial addressing on the user. Its position at $E000 plus the necessary reflections inconveniently breaks up the memory map. Presumably there is a very good reason but I'm baffled Another weird thing is he doesnt specifically ORG the reset/nmi/swi vectors but instead relies on the fact that Mikbug/Minibug is exactly 512 bytes and the reset vectors at the end of the code just happen to align correctly. The slightest code change and its broken! Ta Phil Last edited by Phil__G; 22nd Nov 2020 at 1:26 pm. |
23rd Nov 2020, 6:31 pm | #2 |
Octode
Join Date: May 2010
Location: Grantham, Lincolnshire, UK.
Posts: 1,177
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Re: Motorola 6800 & Mikbug question
It is so MIKBUG can be paged out after boot. One of the wrinkles that needed sorting with my home made Exorciser 40 years ago. After MDOS is booted it uses the whole of the 64K memory.
Are you still using an Exorciser? |
24th Nov 2020, 3:06 am | #3 |
Octode
Join Date: Mar 2011
Location: North Yorkshire, UK.
Posts: 1,119
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Re: Motorola 6800 & Mikbug question
Thanks Woodchips, still not sure, why not load it at $FE00-$FFFF and page it out?
What I dont follow is that it has to end at $FFFF for the reset vector, so why didnt Mike locate it there, instead of relying on a reflection? I dont have an Exorciser I was all 8080/Z80 at the time... Ta Phil |