Subj : Re: The XT that became a 486
To   : kirkspragg
From : hyjinx
Date : Fri Sep 19 2025 19:45:10

 ki>  hy> Al's Geek Lab delivers you -- my IBM 5160, complete with original 256
 ki>  hy> motherboard still in play, delivers you a slice of DooM running at
 ki>  hy> ~20FPS.
 ki>  hy> 
 ki>  hy> Yes, it is running with an upgrade card - an Intel Inboard/386 PC
 ki>  hy> Yes, I rodded the hell out of that too, with a ParrotyError 4MB ram
 ki>  hy> expansion piggyback board and an IODATA PK486 Blue Lightning 3
 ki>  hy> (IBM/Cyrix) CPU which clocks in at 96MHz!
 ki>  hy> 
 ki> Thats so cool, I wonder how much further you can push it!

The next bit of modding is to /reduce/ the clock speed actually. I am going
to change the crystal on the Inboard to 20MHz (down from 32MHz). This means
I'll be able to run the BL3 CPU at 80MHz rather than 96MHz. I think that
80MHz will be better than the 96MHz because of the bus timing on the InBoard.
 
 ki> Is the processor on the piggyback board socketed or not? If it is I wonder
 ki> you could replace it with......

Yes, the CPU is socketed. I don't think there is much more performance to
get, regardless of the CPU - this is because of the 8 bit bus on the IBM
motherboard.

 ki> Also I wonder what performance would be like using a super XT motherboard 
 ki> runs at a higher clock speed than the XT? Bumping the ISA bus speed up to 
 ki> 10Mhz would surely speed things up quite a bit!

Looks like you already understand the above issue. If I changed the DMA
controller and other support chips on the IBM 5160 mainboard, I could likely
get the ISA bust to at least 8MHz. I did so on my 

 ki> ... A toast  --  To a kinder gentler echo
It is indeed a kinder, gentler echo :). Lovely people!


hyjinx // Alistair Ross
Author of 'Back to the BBS' Documentary: https://bit.ly/3tRINeL (YouTube)
alsgeeklab.com

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