Fancy being a Bug-Hunter? Or doing
a bit of Sherlock Holmes work?
Dave Hitchens tried to track down a bug in his Computer Gammon
program from Issue 27 only to find part of the solution.
Can you throw any light on the problem?
Shortly after Computer Gammon was published in Issue
27, the Editor wrote to me and asked if I could sort out a bug.
Apparently, 400/800 owners experienced a problem with the right hand
die jiggling back and forth slightly – something which does not
happen on the XL or XE. It turns out that this is one of those bugs
that does not respond to conventional debugging techniques, in fact
it's real weird!
FIRST THE FIX ...
Before going into the technicalities, let's have the
cure, which involves modifying three lines, and while we're at it
there are a couple of corrections to the scoring routines you might
like to make. The cure for the jiggling is:
Line 1730 – alter the DATA
141,2,208,24,105,20,141,3,208 to
141,3,208,56,233,20,141,2,208.
Line 1760 – alter the 201,70 to 201,90
Line 2440 – alter the 70 to 90
For the scoring routine make the following changes:
Line 8045 – the (N < 6) should be (N < 7)
Line 8046 – the (N > 19) should be (N > 18)
... THEN THE MYSTERY!
Now to the Sherlock Holmes section!
The dice are represented by players 3 and 4,
overwritten by substrings from DICE$ and their horizontal locations
correspond to the value in location 1. Once initialised, this
location holds '70' and is POKEd into memory location 53250. Adding
20 produces the value to POKE into address 53251. Now, because the
program makes multiple use of both players and colour registers, I
introduced three DLI's along with a VBI with the latter pulsing the
colours and cursor etc. The positional instructions for the dice are
contained in this VBI and could not be more simple viz: LDA 1, STA
53250, CLC, ADC#20, STA 53251. The XE accepts this routine and gives
stable dice but the 400 doesn't! The left die is steady as a rock
but the right one dithers.
It turns out that replacing the CLC/ADC instruction
with LDA #90 cures the jiggling. In addition, using SBC #20 with
either CLC or SEC also cures it. Finally, pressing the BREAK key
stops the jiggling but the fault returns when CONT is typed or even
if BASIC is sent into a tight loop (e.g. GOTO 5 where line 5 simply
reads GOTO 5).
A memory location was chosen so as to scroll the dice into view at
the start of the game but its selection does not affect the Bug (I
think it deserves the capital letter!). There is obviously a very
strange interaction taking place when BASIC is running and when this
particular ADC instruction is used. (I tried ADCing a memory
location and the die still jiggled).
A second, and more intermittent, bug which affects
both machines (but which has not been commented on by readers as far
as I know) is an apparent breakdown of the DLI sequencing. It's as
though they go out of sync in a totally random fashion causing
colour flashing and player positioning fluctuations. In particular,
DLI3 (the final one) appears to intermittently affect previously
scanned areas of the TV display. In fact I've seen this on two items
of commercial software so suspected my machine was at fault but I
have since checked it out on two 400's as well as my XE and the
problem appears on all of them. The DLI's are, incidentally, simply
a string of load/stores, no ADC's are used here!
I have tried everything I know to cure this –
resetting the DLI vector in the VBI, using Immediate/deferred VBI's,
using a normal graphics 0 Display List instead of the modified one
etc., etc. It's all to no avail – until I press the BREAK key when,
hey presto, no flashing! As with the dice Bug, however, if you send
BASIC into one of its tight one-line loops, then back comes the
problem.
I should be very interested to hear from anyone who
can shed light on these problems. There appears to be a common
interactive cause, and the effect with the BREAK key, in particular,
must offer a clue to just where the problem lies.
To summarise, the problems are as follows.
1. Player horizontal positions are perfectly stable
on the XE.
2. They are only stable on the 400/800 system if the ADC instruction
is not incorporated.
3. The BREAK key effectively removes the bugs.
4. Bugs return even if BASIC is operating a closed loop.
So, you clever programmers, how about some real
detective work? To help you out, the VBI code for the game is shown.
There's no better way to improve your programming techniques than
sorting out some really obscure bugs! And don't forget, if you solve
it, let me know!
COMPUTER GAMMON – VBI CODE
| PLA |
|
STA (704) |
also used for |
| LDX #6 |
activate |
STA (705) |
dice energisers |
| LDY #11 |
the VBI |
|
|
| LDA #7 |
|
DEC 0 |
|
| JSR (SET) |
|
LDA 0 |
cursor blink |
| RTS |
|
BNE 4 |
routine |
| |
|
LDA #12 |
|
| LDA #0 |
reset DLI |
STA 0 |
|
| STA 512 |
vector |
STA 1702 |
|
| |
|
|
|
| STA 53252 |
set 'dice energisers'
|
LDA #1 |
dice sizes i.e. |
| STA 53253 |
off screen |
STA 53258 |
double width |
| |
|
STA 53259 |
|
| LDA 1700 |
|
|
|
| CLC |
smooth colour change |
LDA 1703 |
|
| |
|
BEQ 10 |
show the energisers |
| ADC #16 |
used in DLI2 |
LDA #56 |
if 'shaking' dice |
| STA 1700 |
|
STA 53252 |
|
| |
|
LDA #116 |
|
| LDA 1 |
|
STA 53253 |
|
| STA 53250 |
horizontal dice |
|
|
| CLC |
locations |
LDA 1 |
scroll the |
| ADC #20 |
|
BEQ 6 |
dice into view |
| STA 53251 |
|
CMP #70 |
when required |
| |
|
BEQ 2 |
|
| LDA 53770 |
rapid colour change |
INC 1 |
|
| STA 1701 |
for DLI3 |
|
|
| |
|
JMP (END) |
End of VBI |
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