Date   : Sat, 12 Aug 2000 00:03:48 +0100
From   : James Fidell <james@...>
Subject: Re: More tape to disc transfer fun: Alien 8

Quoting Mark Usher (mu.list@...):
> 
> > If I find a way to turn the BBC BASIC source into something plain ASCII,
> > I'll post it.
> There's a program BAS2TXT I had a hand in on the BBC Lives.
> http://www.nvg.ntnu.no/bbc/util/BAS2TXT.zip
> 
> Should do the trick quite nicely.

A couple of mods to get it to work under Linux and the result is below.
Obviously the way I broke the decoding process was to write duplicate code,
emulating the way the VIA clocks ran down, doing the self-modification on 
my code and the real code concurrently.  The only particularly awkward issue
is handling the cycle-stretching that happens when the cpu reads/writes page
&FE.  These days it all looks unbelievably straightforward, which is a bit
disappointing given the amount of time I put into this.  Oh well :)

I haven't a clue which of the different versions of the protection
mechanism this was for, but the principle was the same for all of them.
I have disk versions of both Knight's Lore and Alien8 that I converted
from tape sitting here in my copies of my Beeb disks, so it could have been
one of those or something else entirely.

James


   10clockl=&70
   20clockh=&71
   30latch=&72
   40odd=&73
   50?odd=0
   60 
 1000FORpass=0TO3STEP3
 1010P%=&3000:[ OPTpass
 1020.start
 1030 
 1040\ Set clock & latches
 1050 LDA#&FF:STAclockl
 1060 STAclockh:STAlatch
 1070 
 1080\ LDX #0
 1090 
 1100 LDX#0
 1110 JSRtwo
 1120 
 1130\ LDA &Dxx
 1140 
 1150.incr1
 1160 LDA&D00
 1170 JSRfour
 1180 
 1190\ INC &D23
 1200 
 1210 INC&D23
 1220 INCincr3+1
 1230 JSRsix
 1240 
 1250\ EOR &FE68
 1260 
 1270 JSRfour
 1280 EORclockl
 1290 JSRtwo
 1291 JSRzero
 1300 
 1310\ CMP &FE69
 1320 
 1330 JSRfour
 1340 CMPclockh
 1350 JSRtwo
 1351 JSRzero
 1360 
 1370\ ROL &FE68
 1380 
 1390 PHP
 1400 PHA
 1410 JSRfour
 1420 LDAclockl
 1430 ROLA
 1440 STAlatch
 1450 PLA
 1460 PLP
 1470 JSRsix
 1471 JSRzero
 1480 
 1490\ TAY
 1500 
 1510 TAY
 1520 JSRtwo
 1530 
 1540\ LDA &D23
 1550 
 1560 LDA&D23
 1570 JSRfour
 1580 
 1590\ EOR &FE68
 1600 
 1610 JSRfour
 1620 EORclockl
 1630 JSRtwo
 1631 JSRzero
 1640 
 1650\ STA &D1E
 1660 
 1670 STA&D1E
 1680 STAincr2+1
 1690 JSRfour
 1700 
 1710\ TYA
 1720 
 1730 TYA
 1740 JSRtwo
 1750 
 1760\ CLC
 1770 
 1780 CLC
 1790 JSRtwo
 1800 
 1810\ ADC #xx
 1820 
 1830.incr2
 1840 ADC#&FF
 1850 JSRtwo
 1860 
 1870\ DEC &FE69
 1880 
 1890 JSRfour
 1900 DECclockh
 1910 PHP
 1920 PHA
 1930 LDAlatch
 1940 STAclockl
 1950 PLA
 1960 PLP
 1961 JSRzero
 1970 
 1980\ EOR #xx
 1990 
 2000.incr3
 2010 EOR#0
 2020 JSRtwo
 2030 
 2040\ EOR &FE68
 2050 
 2060 JSRfour
 2070 EORclockl
 2080 JSRtwo
 2081 JSRzero
 2090 
 2100\ EOR &FFxx
 2110 
 2120.incr4
 2130 EOR&FF80
 2140 JSRfour
 2150 
 2160\ INC &D23
 2170 
 2180 INC&D23
 2190 INCincr3+1
 2200 JSRsix
 2210 
 2220\ EOR &FE69
 2230 
 2240 JSRfour
 2250 EORclockh
 2260 JSRtwo
 2261 JSRzero
 2270 
 2280\ EOR &D02,X
 2290 
 2300 EOR&D02,X
 2310 JSRfour
 2320 PHP
 2330 PHA
 2340 CPX#&FE
 2350 BCCloop1
 2360 JSRone
 2370.loop1
 2380 PLA
 2390 PLP
 2400 
 2410\ EOR &E00,X
 2420 
 2430 EOR&E00,X
 2440 JSRfour
 2450 
 2460\ EOR &FE68
 2470 
 2480 JSRfour
 2490 EORclockl
 2500 JSRtwo
 2501 JSRzero
 2510 
 2520\ SEC
 2530 
 2540 SEC
 2550 JSRtwo
 2560 
 2570\ ADC &D03
 2580 
 2590 ADC&D03
 2600 JSRfour
 2610 
 2620\ LDY &FE68
 2630 
 2640 JSRfour
 2650 LDYclockl
 2660 JSRtwo
 2661 JSRzero
 2670 
 2680\ STY &FE69
 2690 
 2700 STYclockh
 2710 PHP
 2720 PHA
 2730 LDAlatch
 2740 STAclockl
 2750 PLA
 2760 PLP
 2761 JSRzero
 2770 
 2780\ EOR &xxxx
 2790 
 2800.incr5
 2810 EOR&C400
 2820 JSRfour
 2830 
 2840\ EOR &FE68
 2850 
 2860 JSRfour
 2870 EORclockl
 2880 JSRtwo
 2881 JSRzero
 2890 
 2900\ CLC
 2910 
 2920 CLC
 2930 JSRtwo
 2940 
 2950\ ADC &E01,X
 2960 
 2970 ADC&E01,X
 2980 JSRfour
 2990 PHP
 3000 PHA
 3010 CPX#&FF
 3020 BNEloop2
 3030 JSRone
 3040.loop2
 3050 PLA
 3060 PLP
 3070 
 3080\ INC &D1E
 3090 
 3100 INC&D1E
 3110 INCincr2+1
 3120 JSRsix
 3130 
 3140\ TAY
 3150 
 3160 TAY
 3170 JSRtwo
 3180 
 3190\ LDA &D1E
 3200 
 3210 LDA&D1E
 3220 JSRfour
 3230 
 3240\ EOR &FE68
 3250 
 3260 JSRfour
 3270 EORclockl
 3280 JSRtwo
 3281 JSRzero
 3290 
 3300\ STA &D1E
 3310 
 3320 STA&D1E
 3330 STAincr2+1
 3340 JSRfour
 3350 
 3360\ TYA
 3370 
 3380 TYA
 3390 JSRtwo
 3400 
 3410\ EOR &FE69
 3420 
 3430 JSRfour
 3440 EORclockh
 3450 JSRtwo
 3451 JSRzero
 3460 
 3470\ EOR &FE68
 3480 
 3490 JSRfour
 3500 EORclockl
 3510 JSRtwo
 3511 JSRzero
 3520 
 3530\ STA &E00,X
 3540 
 3550 STA&E00,X
 3560 STA&4E00,X
 3570 JSRfive
 3580 
 3590\ INC &D44
 3600 
 3610 INC&D44
 3620 INCincr5+1
 3630 JSRsix
 3640 
 3650\ INX
 3660 
 3670 INX
 3680 JSRtwo
 3690 
 3700\ BNE &D02
 3710 
 3720 PHP
 3730 PHA
 3740 BEQloop3
 3750 JSRone
 3760.loop3
 3770 JSRtwo
 3780 PLA
 3790 PLP
 3800 BEQloop4
 3810 JMPincr1
 3820.loop4
 3830 
 3840\ INC &D45
 3850 
 3860 INC&D45
 3870 INCincr5+2
 3880 JSRsix
 3890 
 3900\ LDA &D45
 3910 
 3920 LDA&D45
 3930 JSRfour
 3940 
 3950\ CMP #E3
 3960 
 3970 CMP#&E3
 3980 JSRtwo
 3990 
 4000\ BCC &D79
 4010 
 4020 PHP
 4030 PHA
 4040 BCSloop5
 4050 JSRone
 4060.loop5
 4070 JSRtwo
 4080 PLA
 4090 PLP
 4100 BCCloop6
 4110 
 4120\ LDA #C4
 4130 
 4140 LDA#&C4
 4150 JSRtwo
 4160 
 4170\ STA &D45
 4180 
 4190 STA&D45
 4200 STAincr5+2
 4210 JSRfour
 4220 
 4230\ LDA #3
 4240 
 4250.loop6
 4260 JSRtwo
 4270 
 4280\ STA &258
 4290 
 4300 JSRfour
 4310 
 4320\ LDA#4C
 4330 
 4340 JSRtwo
 4350 
 4360\ STA &287
 4370 
 4380 JSRfour
 4390 
 4400\ LDA #87
 4410 
 4420 JSRtwo
 4430 
 4440\ STA &288
 4450 
 4460 JSRfour
 4470 
 4480\ LDA #2
 4490 
 4500 JSRtwo
 4510 
 4520\ STA &289
 4530 
 4540 JSRfour
 4550 
 4560\ LDY #35
 4570 
 4580 LDY#&35
 4590 JSRtwo
 4600 
 4610\ LDA &D940,Y
 4620 
 4630.loop8
 4640 JSRfour
 4650 
 4660\ STA &200,Y
 4670 
 4680 JSRfive
 4690 
 4700\ DEY
 4710 
 4720 DEY
 4730 JSRtwo
 4740 
 4750\ BPL &D8F
 4760 
 4770 PHP
 4780 PHA
 4790 BMIloop7
 4800 JSRone
 4810.loop7
 4820 JSRtwo
 4830 PLA
 4840 PLP
 4850 BPLloop8
 4860 
 4870\ LDA &D23
 4880 
 4890 LDA&D23
 4900 JSRfour
 4910 
 4920\ EOR &FE68
 4930 
 4940 JSRfour
 4950 EORclockl
 4960 JSRtwo
 4961 JSRzero
 4970 
 4980\ STA &D23
 4990 
 5000 STA&D23
 5010 STAincr3+1
 5020 JSRfour
 5030 
 5040\ INC &D28
 5050 
 5060 INC&D28
 5070 INCincr4+1
 5080 JSRsix
 5090 
 5100\ INC &D23
 5110 
 5120 INC&D23
 5130 INCincr3+1
 5140 JSRsix
 5150 
 5160\ INC &D03
 5170 
 5180 INC&D03
 5190 INCincr1+1
 5200 JSRsix
 5210 
 5220\ LDA &D03
 5230 
 5240 LDA&D03
 5250 JSRfour
 5260 TAY:STA&7C00,Y
 5270 
 5280\ CMP #0
 5290 
 5300 CMP#0
 5310 JSRtwo
 5320 
 5330\ BEQ &DB4
 5340 
 5350 JSRtwo
 5360 BEQfinish
 5370 
 5380\ JMP &D02
 5390 
 5400 JSRthree
 5410 JMPincr1
 5420 
 5430.finish
 5440 RTS
 5450 
 5460.one
 5470 PHP
 5480 PHA
 5490 INCodd
 5500 LDAodd
 5510 CMP#2
 5520 BNEloopA
 5530 JSRzero
 5550 JSRtwo
 5560.loopA
 5570 PLA
 5580 PLP
 5590 RTS
 5600 
 5610.two
 5620 PHP
 5630 PHA
 5640 SEC
 5650 LDAclockl
 5660 SBC#1
 5670 STAclockl
 5680 LDAclockh
 5690 SBC#0
 5700 STAclockh
 5710 PLA
 5720 PLP
 5730 RTS
 5740 
 5750.three
 5760 JSRone
 5770 JSRtwo
 5780 RTS
 5790 
 5800.four
 5810 JSRtwo
 5820 JSRtwo
 5830 RTS
 5840 
 5850.five
 5860 JSRfour
 5870 JSRone
 5880 RTS
 5890 
 5900.six
 5910 JSRfour
 5920 JSRtwo
 5930 RTS
 5940 
 5950.ten
 5960 JSRfive
 5970 JSRfive
 5980 RTS
 5990 
 5991.zero
 5992 PHP
 5993 PHA
 5994 LDA#0
 5995 STAodd
 5996 PLA
 5997 PLP
 5998 RTS
 5999 
 6000]:NEXT

-- 
 "Yield to temptation --             | Consultancy: james@...          
  it may not pass your way again"    | http://www.cloud9.co.uk/james
                                     |
        - Lazarus Long               |              James Fidell