Date   : Sun, 17 Feb 2008 19:40:03 +0000
From   : jgh@... (Jonathan Graham Harston)
Subject: BBC FPGA Boots to BASIC... almost...

Mark McDougall wrote:
> Michael Foot wrote:
> 
> > They need to return &FE instead. ie. the high part of the address.
 
Testing on real hardware shows all unused I/O addresses return
&FE, not just those at &FExx:
 
=>*mdump -80 FFFC00+300
FFFC00 FE FE FE FE FE FE FE FE FE FE FE FE FE FE FE FE ................
FFFC10 FE FE FE FE FE FE FE FE FE FE FE FE FE FE FE FE ................
[snip]
FFFCF0 FE FE FE FE FE FE FE FE FE FE FE FE FE FE FE FE ................
FFFD00 FE FE FE FE FE FE FE FE FE FE FE FE FE FE FE FE ................
FFFD10 FE FE FE FE FE FE FE FE FE FE FE FE FE FE FE FE ................
[snip]
FFFDF0 FE FE FE FE FE FE FE FE FE FE FE FE FE FE FE FE ................
FFFE00 40 76 40 76 40 76 60 76 08 00 08 00 08 00 08 00 @v at v@...
FFFE10 00 7C 7C 40 60 60 00 00 B9 C1 C0 C0 B9 C1 C0 C0 .||@...@@...@@
FFFE20 00 60 00 60 60 00 60 60 00 4B 01 11 7C 7C 00 00 .`.``.``.K..||..
FFFE30 08 08 08 08 0B 0B 0B 0B 00 00 00 00 00 00 00 7E ...............~
FFFE40 3B 81 CF 7F B8 20 0E 27 48 4F FE 40 04 00 D3 81 ;.O.8 .'HO. at ..S.
FFFE50 3B 81 CF 7F 69 04 0E 27 79 FA FE 40 04 00 D3 81 ;.O.i..'yz. at ..S.
FFFE60 FE 00 00 FE 07 1F FE 5F D4 A3 0F 00 0E 00 80 00 ......._T#......
FFFE70 FE 00 00 FE EB 2B FE 5F AF 4F 0F 00 0E 00 80 00 ....k+._/O......
FFFE80 00 00 7E 7E 00 60 60 00 40 60 60 00 60 00 40 40 ..~~.``.@``.`.@@
FFFE90 60 00 18 60 60 40 60 60 00 00 60 60 00 00 00 00 `..``@...
FFFEA0 10 04 00 00 10 04 00 00 10 04 00 00 10 04 00 00 ................
FFFEB0 10 04 00 00 10 04 00 00 10 04 00 00 10 04 00 00 ................
FFFEC0 00 00 3C 3C 00 00 00 7E 3C 00 66 1C 66 60 00 1C ..<<...~<.f.f`..
FFFED0 00 3C 60 00 18 3C 60 60 00 00 00 00 00 00 00 7E .<`..<``.......~
FFFEE0 FE FE FE FE FE FE FE FE FE FE FE FE FE FE FE FE ................
FFFEF0 FE FE FE FE FE FE FE FE FE FE FE FE FE FE FE FE ................
 
One thing to look out for is that the BBCs return &FE and the
Masters return &FF on unused ports. Some software depends on b7
being 1 when hardware is absent (eg detecting Tube or hard drive
hardware).
 
-- 
J.G.Harston - jgh@...                - mdfs.net/User/JGH
HADFS System Resources - http://mdfs.net/Software/HADFS