[OpenBIOS] Incorrect implementation of b?branch?

[Mark Cave-Ayland]

Hi everyone,

I've been spending some time over the past few weeks trying to get a 
Sparc64/Solaris disk image to boot under qemu, and as far as I can tell, 
the first reason that the code terminates incorrectly is because 
b?branch doesn't work.

This can be demonstrated fairly easily using the following couple of 
Fcode programs. Note that while I'm working on Sparc64, I see the same 
issue on x86 OpenBIOS too.


Example 1: b?branch with -1 on stack: ignore branch & abort

CC		\ offset16
10		\ b(lit)
FF FF FF FF	\ (-1)
14 00 04	\ b?branch
02 16		\ if false, abort
9D		\ if true, display stack
00		\ end

0 > true to ?fcode-verbose  ok
0 > here CC10FFFFFFFF1400 /n allot over !  ok
1 > here 0402169D00000000 /n allot over ! drop  ok
1 > 1 byte-load
byte-load: evaluating fcode at 0xffd9d128
fcode-table at 0xffdb7f90
offset16 [ 0xcc ]
b(lit) [ 0x10 ]
b?branch [ 0x14 ]
abort [ 0x216 ]

byte-load: exception caught!
  ok


Example 2: b?branch with 0 on stack: skip abort, display stack

CC		\ offset16
10		\ b(lit)
00 00 00 00	\ (0)
14 00 04	\ b?branch
02 16		\ if false, abort
9D		\ if true, display stack
00		\ end

0 > true to ?fcode-verbose  ok
0 > here CC10000000001400 /n allot over !  ok
1 > here 0402169D00000000 /n allot over ! drop  ok
1 > 1 byte-load
byte-load: evaluating fcode at 0xffd9d128
fcode-table at 0xffdb7f90
offset16 [ 0xcc ]
b(lit) [ 0x10 ]
b?branch [ 0x14 ]
abort [ 0x216 ]

byte-load: exception caught!
  ok


Okay, so did anyone notice the difference?! This led me to look into the 
definition of b?branch in forth/device/fcode.fs:


\ b?branch ( continue? -- )
\   Conditional branch FCode. Followed by FCode-offset.

: b?branch
   ['] do?branch ,
   fcode-offset 0< if \ if we jump backwards, we can forsee where it goes
     resolve-dest
   else
     here
     0 ,
   then
   ; immediate


By adding some extra debugging into b?branch, I can see that for the 
above Fcode, do?branch is *never* called, although it correct takes the 
"else" branch listed above.

I'm still reasonably new to Forth, but AIUI, all the code above does is 
allocate one cell to store the xt of do?branch, and in the case of a 
forward branch, push the here location onto the stack and then store 0 
into another allocated cell. This agrees with my investigation that 
shows that do?branch is never actually called.

Given that this code is also missing a section that will read and 
discard Fcode-offset-2 bytes for a forward branch, I'm inclined to think 
that this code is just wrong. Can anyone else with any experience with 
this code comment upon this?


ATB,

Mark.

-- 
Mark Cave-Ayland
Sirius Corporation - The Open Source Experts
http://www.siriusit.co.uk
T: +44 870 608 0063