Amstrad PCW 8256 and 3.5 DS2D Drives

Chuck Guzis cclist at
Tue Jul 10 12:21:24 CDT 2007

I recently picked up an Amstrad 8256 for its floppy drive (Hint:  
replace the drive belt with a cheap polyurethane "elastiband" type of 
"rubber" band.  It works great.)  That left me with a machine that 
would either hit the recyclers or that I could keep around.

When I looked at the basic box--4MHz Z80, graphics, 512K of RAM...I 
couldn't bear to toss it out without at least playing with it a bit.  
I installed a Teac FD235F 720K drive in place of the original one and 
then contemplated what I was going to do for a boot diskette.  None 
came with the machine.

Digging through my collection turned up a 3.5" CP/M 3.0 boot diskette 
for a PCW 9512 (DisKit), but simply trying to boot from it got me 
nowhere. (Beep Beep Beep).  I found that the checksum of the boot 
sector identified the target boot machine.  My sample checksummed to 
0x01, where my documentation said that the PCW 8256 required a 
checksum of 0xff.  Decrementing the "fixup byte" at offset 0x0f in 
the boot sector by 3 from 0xD1 to 0xCF did the trick.  The lowly 
Joyce now boots CP/M.  I've not seen this trick documented, so I 
thought I'd report it here.  The easiest thing would have been to 
change the boot ROM code, but that appears to be buried in a gate 
array and not a regular EPROM.

I note that the PCW uses the RST 7 vector for interrupts, which 
conflicts with the breakpoint interrupt used by DDT and ZSID.  
Fortunately, there appears to be a patch for that.   

While it might be fun to get the PCW to run MP/M II, my interest has 
waned a bit, so the thing will now go back onto the shelf.  Without a 
regular printer or serial port, an anemic PSU and a horrible 
keyboard,  it's going to be of limited utility anyway--for now.


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