----------------Outpost---------------- A 4am crack 2015-12-12 -------------------. updated 2020-06-24 |___________________ Name: Outpost Genre: arcade Year: 1981 Authors: Tom McWilliams Publisher: Sirius Software Media: single-sided 5.25-inch floppy OS: custom Previous cracks: two different file cracks, both uncredited Similar cracks: #315 Beer Run ~ Chapter 0 In Which Various Automated Tools Fail In Interesting Ways COPYA immediate disk read error Locksmith Fast Disk Backup unable to read any track EDD 4 bit copy (no sync, no count) hangs during boot Copy ][+ nibble editor track 0 has some 4-4 encoded data other tracks are unreadable Disk Fixer nope (can't read 4-4 encoded tracks) Why didn't COPYA work? not a 16-sector disk Why didn't Locksmith FDB work? ditto Why didn't my EDD copy work? I don't know. Could be a nibble check during boot. Could be that the data is loaded from half tracks. Could be both, or neither. Next steps: 1. Trace the boot 2. Capture the game in memory 3. Write it out to a standard disk with some kind of fastloader ~ Chapter 1 In Which We Find A Very Unfriendly "Do Not Disturb" Sign [S6,D1=original disk] [S5,D1=my work disk] ]PR#5 CAPTURING BOOT0 ...reboots slot 6... ...reboots slot 5... SAVING BOOT0 ]BLOAD BOOT0,A$800 ]CALL -151 *801L ; display hi-res graphics page ; (uninitialized) 0801- 8D 50 C0 STA $C050 0804- 8D 52 C0 STA $C052 0807- 8D 54 C0 STA $C054 080A- 8D 57 C0 STA $C057 ; get slot (x16) 080D- A6 2B LDX $2B ; a counter? or an address? 080F- A9 04 LDA #$04 0811- 85 11 STA $11 0813- A0 00 LDY #$00 0815- 84 10 STY $10 ; look for custom prologue ("DD AD DA") 0817- BD 8C C0 LDA $C08C,X 081A- 10 FB BPL $0817 081C- C9 DD CMP #$DD 081E- D0 F7 BNE $0817 0820- BD 8C C0 LDA $C08C,X 0823- 10 FB BPL $0820 0825- C9 AD CMP #$AD 0827- D0 F3 BNE $081C 0829- BD 8C C0 LDA $C08C,X 082C- 10 FB BPL $0829 082E- C9 DA CMP #$DA 0830- D0 EA BNE $081C ; read 4-4 encoded data immediately ; (no address field, no sector numbers) 0832- BD 8C C0 LDA $C08C,X 0835- 10 FB BPL $0832 0837- 38 SEC 0838- 2A ROL 0839- 85 0E STA $0E 083B- BD 8C C0 LDA $C08C,X 083E- 10 FB BPL $083B 0840- 25 0E AND $0E ; ($10) is an address, initialized at ; $080F as $0400 (yes, the text page) 0842- 91 10 STA ($10),Y 0844- C8 INY 0845- D0 EB BNE $0832 0847- E6 11 INC $11 0849- A5 11 LDA $11 ; loop until we hit page 8 (i.e. we're ; filling $0400..$07FF) 084B- C9 08 CMP #$08 084D- D0 E3 BNE $0832 084F- BD 80 C0 LDA $C080,X ; clear $0900..$BFFF in main memory 0852- A9 09 LDA #$09 0854- 85 01 STA $01 0856- A9 00 LDA #$00 0858- 85 00 STA $00 085A- A8 TAY 085B- A2 B7 LDX #$B7 085D- 91 00 STA ($00),Y 085F- C8 INY 0860- D0 FB BNE $085D 0862- E6 01 INC $01 0864- CA DEX 0865- D0 F6 BNE $085D ; calculate a checksum of page 8 (this ; code right here) 0867- 8A TXA 0868- E8 INX 0869- F0 06 BEQ $0871 086B- 5D 00 08 EOR $0800,X 086E- 4C 68 08 JMP $0868 ; use the stack pointer (!) to keep a ; copy of that checksum 0871- AA TAX 0872- 9A TXS ; calculate another checksum of zero ; page 0873- A2 00 LDX #$00 0875- 8A TXA 0876- 55 00 EOR $00,X 0878- E8 INX 0879- D0 FB BNE $0876 ; get slot (x16) again 087B- A6 2B LDX $2B ; jump to the code we just read into ; the text page 087D- 4C 00 04 JMP $0400 Well that's lovely. I need to interrupt the boot at $087D, but if I do, it will modify the checksum that ends up in the stack pointer (which is a great place to stash a checksum as long as you never use PHA, PLA, PHP, PLP, JSR, RTS, or RTI). It's also wiping main memory, including the place I usually put my boot trace callbacks (around $9700). So, a three-pronged attack: 1. Relocate the code to $0900. Most of it uses relative branching already, except for one JMP at $086E, which I can patch. The code will still run, but I'll be able to patch it without altering the checksum. 2. Disable the memory wipe at $095D. 3. Patch the code at $097D to jump to a routine under my control. ~ Chapter 2 In Which Nothing Happens, Inhospitably *9600 *97FF 97FF- 20 The initial checksum of boot0 is $20. *C500G ... ]CALL -151 *9600 *97FF 97FF- 00 The second checksum, which gets pushed twice to the stack at $0470, is $00. ~ Chapter 4 In Which Half A Track Is Better Than None Continuing the boot trace at $0472... *C500G ... ]BLOAD BOOT1 0400-07FF,A$2400 ]CALL -151 *2472L 2472- A0 03 LDY #$03 2474- 20 00 05 JSR $0500 *2500L 2500- 20 DC 04 JSR $04DC *24DCL ; advance drive head by one phase ; (a.k.a. a half track) 24DC- E6 0C INC $0C 24DE- A5 0C LDA $0C 24E0- 29 03 AND #$03 24E2- 0A ASL 24E3- 05 2B ORA $2B 24E5- AA TAX 24E6- BD 81 C0 LDA $C081,X 24E9- 20 F8 04 JSR $04F8 24EC- BD 80 C0 LDA $C080,X 24EF- 20 F8 04 JSR $04F8 ; loop a number of times (given in the ; Y register on entry) 24F2- 88 DEY 24F3- D0 E7 BNE $24DC 24F5- A6 2B LDX $2B 24F7- 60 RTS 24F8- 8D 50 C0 STA $C050 24FB- A9 40 LDA #$40 24FD- 4C A8 FC JMP $FCA8 We started on track 0 and advanced the drive head by 3 phases, so now we're on track 1.5. Continuing from $0503... ; save X, display "Outpost" on hi-res ; screen (not shown), restore X 2503- 86 2B STX $2B 2505- A9 00 LDA #$00 2507- A2 0F LDX #$0F 2509- A0 18 LDY #$18 250B- 20 60 05 JSR $0560 250E- A6 2B LDX $2B 2510- 60 RTS Continuing from $0477... *2477L ; get target memory page from an array ; at $05F0 2477- A4 0E LDY $0E 2479- B9 F0 05 LDA $05F0,Y ; when page = 0, jump to next stage ; at $0520, otherwise continue at $0481 247C- D0 03 BNE $2481 247E- 4C 20 05 JMP $0520 2481- 20 90 04 JSR $0490 *2490L ; sector count (4-4 encoded tracks can ; only hold $0C pages worth of data) 2490- 85 05 STA $05 2492- 18 CLC 2493- A9 0C LDA #$0C 2495- 85 06 STA $06 2497- A0 00 LDY #$00 2499- 84 04 STY $04 ; custom prologue "DD AD DA" 249B- BD 8C C0 LDA $C08C,X 249E- 10 FB BPL $249B 24A0- C9 DD CMP #$DD 24A2- D0 F7 BNE $249B 24A4- BD 8C C0 LDA $C08C,X 24A7- 10 FB BPL $24A4 24A9- C9 AD CMP #$AD 24AB- D0 F3 BNE $24A0 24AD- BD 8C C0 LDA $C08C,X 24B0- 10 FB BPL $24AD 24B2- C9 DA CMP #$DA 24B4- D0 EA BNE $24A0 ; now read 4-4 encoded data into ($04) 24B6- BD 8C C0 LDA $C08C,X 24B9- 10 FB BPL $24B6 24BB- 8D 57 C0 STA $C057 24BE- 38 SEC 24BF- 2A ROL 24C0- 8D 50 C0 STA $C050 24C3- 85 0F STA $0F 24C5- BD 8C C0 LDA $C08C,X 24C8- 10 FB BPL $24C5 24CA- 25 0F AND $0F 24CC- 91 04 STA ($04),Y 24CE- C8 INY 24CF- D0 E5 BNE $24B6 ; increment target page 24D1- E6 05 INC $05 ; decrement count 24D3- C6 06 DEC $06 ; Loop back to read more. Note: this ; goes directly to data read routine, ; not the prologue match routine. There ; is only one prologue per track. 24D5- D0 DF BNE $24B6 24D7- 60 RTS Continuing from $0484... *2484L ; sets Y=2 and falls through to drive ; head advance routine, so this will ; skip ahead 2 phases = 1 whole track, ; so we're still on half tracks but now ; 2.5, 3.5, 4.5, &c. 2484- 20 D8 04 JSR $04D8 ; show hi-res screen, increment index ; into page array, and jump back to ; read the next track 2487- 8D 50 C0 STA $C050 248A- E6 0E INC $0E 248C- 4C 77 04 JMP $0477 Here is the target page table (accessed at $0479): *25F0. 25F0- 08 14 40 4C 58 64 70 7C 25F8- 88 00 Each call to $0490 reads $0C sectors, so we're filling $0800..$1FFF, skipping hi-res screen 1 (initialized earlier with the graphical "Outpost" loading screen), then filling $4000..$93FF. Once the page array is exhausted, $047E jumps to $0520 for the next boot stage. To sum up: - We're reading data from consecutive half tracks (1.5, 2.5, 3.5, &c.) - Each track has $0C pages of data in a custom (non-sector-based) format - We're using $0800..$93FF in main memory (hi-res screen 1 was drawn earlier, then the rest is read directly from disk) - Nothing in this read loop relies on the checksum we stashed in the stack pointer or the later checksum we pushed twice to the stack - $047E exits via $0520 Let's capture it. ~ Chapter 5 In Which Things Have Been Made As Difficult As Possible For Us *9600 A quick inspection of memory confirms that $0800..$93FF have changed, and the rest are untouched (except the text page, but I knew that). According to "Inside the Apple //e" (pp. 296-8), $C311 copies data from main memory to aux memory and back. (Aux memory is what you get by having an 80-column card, 128K instead of 64.) The routine itself takes 4 parameters: ($3C/$3D) starting address ($3E/$3F) ending address ($42/$43) destination address in the other memory bank carry bit set for main->aux copy, or clear for aux->main copy Thus, to copy $0800..$93FF to auxiliary memory: 0300- A9 00 LDA #$00 0302- 85 3C STA $3C 0304- 85 42 STA $42 0306- A9 08 LDA #$08 0308- 85 3D STA $3D 030A- 85 43 STA $43 030C- A9 FF LDA #$FF 030E- 85 3E STA $3E 0310- A9 93 LDA #$93 0312- 85 3F STA $3F 0314- 38 SEC 0315- 4C 11 C3 JMP $C311 *300G ; reboot to my work disk *C500G ... ]CALL -151 And copy $0800..$93FF from auxiliary memory back to main memory, I only need to change the "SEC" to "CLC" at $0314: 0300- A9 00 LDA #$00 0302- 85 3C STA $3C 0304- 85 42 STA $42 0306- A9 08 LDA #$08 0308- 85 3D STA $3D 030A- 85 43 STA $43 030C- A9 FF LDA #$FF 030E- 85 3E STA $3E 0310- A9 93 LDA #$93 0312- 85 3F STA $3F 0314- 18 CLC 0315- 4C 11 C3 JMP $C311 *300G *BSAVE OBJ,A$800,L$8C00 Continuing from $0520... *BLOAD BOOT1 0400-07FF,A$2400 *2520L ; turn off drive motor 2520- BD 88 C0 LDA $C088,X 2523- 20 D0 07 JSR $07D0 *27D0L ; calculate a simple one-byte checksum ; on the entire game code (minus hi-res ; graphics screen 1) to ensure the game ; code has not been tampered with 27D0- A0 00 LDY #$00 27D2- 84 04 STY $04 27D4- A9 08 LDA #$08 27D6- 85 05 STA $05 27D8- A9 00 LDA #$00 27DA- 51 04 EOR ($04),Y 27DC- C8 INY 27DD- D0 FB BNE $27DA 27DF- E6 05 INC $05 27E1- A6 05 LDX $05 ; skip from $2000 to $4000 27E3- E0 20 CPX #$20 27E5- D0 F3 BNE $27DA 27E7- 06 05 ASL $05 ; continue calculating checksum in the ; accumulator 27E9- 51 04 EOR ($04),Y 27EB- C8 INY 27EC- D0 FB BNE $27E9 27EE- E6 05 INC $05 27F0- A6 05 LDX $05 27F2- E0 94 CPX #$94 27F4- D0 F3 BNE $27E9 27F6- A8 TAY ; if checksum fails, it's off to The ; Badlands with you! 27F7- D0 87 BNE $2780 27F9- 60 RTS Continuing from $0526... *2526L ; get those checksum values we pushed ; to the stack at $0470 and start ; fiddling with them 2526- 68 PLA ;A=$00 2527- AA TAX ;X=$00 2528- 68 PLA ;A=$00 2529- 38 SEC 252A- 69 7E ADC #$7E ;A=$7F 252C- 48 PHA ;S+$7F 252D- 8A TXA ;A=$00 252E- 18 CLC 252F- E9 00 SBC #$00 2531- 48 PHA ;S+$FF 2532- 38 SEC 2533- 69 36 ADC #$36 ;A=$36 2535- 85 00 STA $00 2537- 38 SEC 2538- E9 36 SBC #$36 ;A=$00 253A- 85 01 STA $01 ($00) points to $0036 now. 253C- A8 TAY ;Y=$00 253D- 68 PLA ;A=$FF 253E- 48 PHA Still $7F/$FF on the stack. 253F- 18 CLC 2540- 69 64 ADC #$64 ;A=$63 2542- 91 00 STA ($00),Y zp$36 = $63 now. 2544- C8 INY ;Y=$01 2545- 38 SEC 2546- 69 00 ADC #$00 ;A=$64 2548- 91 00 STA ($00),Y zp$37 = $64 now. 254A- A9 00 LDA #$00 254C- 85 00 STA $00 ($00) points to $0000 now. 254E- 68 PLA ;A=$FF 254F- 48 PHA Still $7F/$FF on the stack. 2550- 91 00 STA ($00),Y zp$00 = $FF now. ($00) points to $00FF. 2552- C8 INY ;Y=$01 2553- 38 SEC 2554- E9 08 SBC #$08 ;A=$F7 2556- 91 00 STA ($00),Y $0100 = $F7 now. ($36) points to $6463. $0100 = $F7. The game starts at $8000. If I reproduce the initializations from this obfuscated routine at $0520, I should be able to run the game from the monitor. I need to do this all at once, since returning to the monitor will reset $36 and possibly $100 as well. *36:63 64 N 100:F7 N 8000G ...crashes... I'm missing something. Maybe a callback to the RWTS on the text page? I've seen other Sirius games do that. *C500G ... ]CALL -151 *BLOAD OBJ *BLOAD BOOT1 0400-07FF,A$9400 *36:63 64 N 100:F7 N 400<9400.97FFM N 8000G ...crashes... Still no luck. Maybe some secondary protection in the game code? Or even a secondary loader? (I've seen both in other Sirius games.) Sigh. Let's start tracing through the code at $8000. ~ Chapter 6 And One More Thing *BLOAD OBJ *8000L 8000- 20 00 81 JSR $8100 *8100L ; harmless 8100- A9 C0 LDA #$C0 8102- 85 45 STA $45 8104- A9 00 LDA #$00 8106- 85 58 STA $58 8108- 85 83 STA $83 810A- 85 84 STA $84 810C- 85 85 STA $85 810E- 85 86 STA $86 8110- 85 87 STA $87 8112- 85 88 STA $88 8114- A9 01 LDA #$01 8116- 85 8B STA $8B 8118- 85 60 STA $60 811A- 4C 30 81 JMP $8130 *8130L ; harmless 8130- 8D 3F 76 STA $763F 8133- A9 00 LDA #$00 8135- 85 80 STA $80 8137- 85 81 STA $81 8139- 85 82 STA $82 813B- 85 8C STA $8C 813D- 4C C0 8E JMP $8EC0 *8EC0L ; hmm 8EC0- 20 00 8F JSR $8F00 8EC3- B0 0B BCS $8ED0 8EC5- 20 0B 8F JSR $8F0B 8EC8- B0 06 BCS $8ED0 8ECA- 4C 70 07 JMP $0770 8ECD- 00 BRK 8ECE- 00 BRK 8ECF- 00 BRK 8ED0- BD 88 C0 LDA $C088,X 8ED3- 60 RTS *8F00L ; turn on boot slot drive motor ; (DEFINITELY NOT HARMLESS) 8F00- A6 2B LDX $2B 8F02- BD 89 C0 LDA $C089,X ; advance drive by 2 phases (=1 track) 8F05- A0 02 LDY #$02 8F07- 20 DC 04 JSR $04DC 8F0A- EA NOP 8F0B- A9 00 LDA #$00 8F0D- 85 01 STA $01 8F0F- A8 TAY ; look for prologue, "D5 AA AD" 8F10- BD 8C C0 LDA $C08C,X 8F13- 10 FB BPL $8F10 8F15- C9 D5 CMP #$D5 8F17- D0 F7 BNE $8F10 8F19- BD 8C C0 LDA $C08C,X 8F1C- 10 FB BPL $8F19 8F1E- C9 AA CMP #$AA 8F20- D0 F3 BNE $8F15 8F22- BD 8C C0 LDA $C08C,X 8F25- 10 FB BPL $8F22 8F27- C9 AD CMP #$AD 8F29- D0 EA BNE $8F15 ; count nibbles until epilogue, "DE AA" 8F2B- C8 INY 8F2C- D0 04 BNE $8F32 8F2E- E6 01 INC $01 8F30- F0 17 BEQ $8F49 8F32- BD 8C C0 LDA $C08C,X 8F35- 10 F9 BPL $8F30 8F37- C9 DE CMP #$DE 8F39- D0 F0 BNE $8F2B 8F3B- BD 8C C0 LDA $C08C,X 8F3E- 10 F9 BPL $8F39 8F40- C9 AA CMP #$AA 8F42- D0 E7 BNE $8F2B ; if >= $0C00 nibbles between prologue ; and epilogue, carry is set on exit 8F44- A5 01 LDA $01 8F46- C9 0C CMP #$0C 8F48- 60 RTS 8F49- 38 SEC 8F4A- 60 RTS Returning to $8EC0... *8EC0L ; count nibbles 8EC0- 20 00 8F JSR $8F00 ; carry set = success, exit via $8ED0 8EC3- B0 0B BCS $8ED0 ; count nibbles again (but stay on the ; same track) 8EC5- 20 0B 8F JSR $8F0B ; carry set = success, exit via $8ED0 8EC8- B0 06 BCS $8ED0 ; failure --> The Badlands 8ECA- 4C 70 07 JMP $0770 8ECD- 00 BRK 8ECE- 00 BRK 8ECF- 00 BRK ; success path, turn off drive motor ; and return gracefully 8ED0- BD 88 C0 LDA $C088,X 8ED3- 60 RTS I should be able to put an "RTS" at $8EC0 to disable this secondary protection altogether. *BLOAD BOOT1 0400-07FF,A$9400 ; set $36/$37, $0100, copy RWTS to text ; page, disable secondary protection, ; and jump to the game entry point *36:63 64 N 100:F7 N 400<9400.97FFM N 8EC0:60 N 8000G ...game works, and it is glorious... I didn't bother setting zp$01 because the secondary protection overwrites it. The game doesn't appear to care about zp$01 is after it's checked at $8F46. It also doesn't seem to care about $00. However, I tried not setting $36/$37 and the graphics glitched out, so that vector is being used for something. I might be able to reduce this further, but $0400..$93FF is a nice round number (exactly 9 tracks on a 16-sector disk), so let's move on. ~ Chapter 7 In Which We Step, Ever So Gently, Into The 21st Century To reproduce the original disk's boot experience as faithfully as possible, I decided against releasing this as a file crack. The original disk displays the graphical title screen during boot. In fact, it *only* displays it during boot, then never again. Classic cracks often didn't include the title screen, because it was the 80s and 8192 bytes was expensive. The social mores of the classic crackers allowed for discarding title screens altogether in pursuit of the smallest possible file crack. I have all the game code. I know how to initialize it and call it. Now to write it all to disk. (We'll worry about reading it back in just a minute.) [S6,D1=blank formatted disk] [S5,D1=my work disk] ]PR#5 ... ]CALL -151 ; page count (decremented) 0300- A9 90 LDA #$90 0302- 85 FF STA $FF ; logical sector (incremented) 0304- A9 00 LDA #$00 0306- 85 FE STA $FE ; call RWTS to write sector 0308- A9 03 LDA #$03 030A- A0 88 LDY #$88 030C- 20 D9 03 JSR $03D9 ; increment logical sector, wrap around ; from $0F to $00 and increment track 030F- E6 FE INC $FE 0311- A4 FE LDY $FE 0313- C0 10 CPY #$10 0315- D0 07 BNE $031E 0317- A0 00 LDY #$00 0319- 84 FE STY $FE 031B- EE 8C 03 INC $038C ; convert logical to physical sector 031E- B9 40 03 LDA $0340,Y 0321- 8D 8D 03 STA $038D ; increment page to write 0324- EE 91 03 INC $0391 ; loop until done with all $90 pages 0327- C6 FF DEC $FF 0329- D0 DD BNE $0308 032B- 60 RTS *340.34F ; logical to physical sector mapping 0340- 00 07 0E 06 0D 05 0C 04 0348- 0B 03 0A 02 09 01 08 0F *388.397 ; RWTS parameter table, pre-initialized ; with slot 6, drive 1, track $01, ; sector $00, address $1400, and RWTS ; write command ($02) 0388- 01 60 01 00 01 00 FB F7 0390- 00 14 00 00 02 00 00 60 *BSAVE MAKE,A$300,L$98 *BLOAD BOOT1 0400-07FF,A$1400 *BLOAD OBJ,A$1800 *300G ; write game to disk Now I have the entire game on tracks $01-$09 of a standard 16-sector disk. To read it back as quickly as possible, I'll use qkumba's "0boot" bootloader. ~ Chapter 8 0boot 0boot lives on track $00, just like me. Sector $00 (boot0) reuses the disk controller ROM routine to read sector $0E (boot1). Boot0 creates a few data tables, copys boot1 to zero page, modifies it to accomodate booting from any slot, and jumps to it. Boot0 is loaded at $0800 by the disk controller ROM routine. ; tell the ROM to load only this sector ; (we'll do the rest manually) 0800- [01] ; The accumulator is $01 after loading ; sector $00, or $03 after loading ; sector $0E. We don't need to preserve ; the value, so we just shift the bits ; to determine whether this is the ; first or second time we've been here. 0801- 4A LSR ; second run -- we've loaded boot1, so ; skip to boot1 initialization routine 0802- D0 0E BNE $0812 ; first run -- increment the physical ; sector to read (this will be the next ; sector under the drive head, so we'll ; waste as little time as possible ; waiting for the disk to spin) 0804- E6 3D INC $3D ; X holds the boot slot (x16) -- ; munge it into $Cx format (e.g. $C6 ; for slot 6, but we need to accomodate ; booting from any slot) 0806- 8A TXA 0807- 4A LSR 0808- 4A LSR 0809- 4A LSR 080A- 4A LSR 080B- 09 C0 ORA #$C0 ; push address (-1) of the sector read ; routine in the disk controller ROM 080D- 48 PHA 080E- A9 5B LDA #$5B 0810- 48 PHA ; "return" via disk controller ROM, ; which reads boot1 into $0900 and ; exits via $0801 0811- 60 RTS ; Execution continues here (from $0802) ; after boot1 code has been loaded into ; $0900. This works around a bug in the ; CFFA 3000 firmware that doesn't ; guarantee that the Y register is ; always $00 at $0801, which is exactly ; the sort of bug that qkumba enjoys ; uncovering. 0812- A8 TAY ; munge the boot slot, e.g. $60 -> $EC ; (to be used later) 0813- 8A TXA 0814- 09 8C ORA #$8C ; Copy the boot1 code from $0901..$09FF ; to zero page. ($0900 holds the 0boot ; version number. This is version 1. ; $0000 is initialized later in boot1.) 0816- BE 00 09 LDX $0900,Y 0819- 96 00 STX $00,Y 081B- C8 INY 081C- D0 F8 BNE $0816 ; There are a number of places in boot1 ; that need to hit a slot-specific soft ; switch (read a nibble from disk, turn ; off the drive, &c). Rather than the ; usual form of "LDA $C08C,X", we will ; use "LDA $C0EC" and modify the $EC ; byte in advance, based on the boot ; slot. $00F5 is an array of all the ; places in the boot1 code that need ; this adjustment. 081E- C8 INY 081F- B6 F5 LDX $F5,Y 0821- 95 00 STA $00,X 0823- D0 F9 BNE $081E ; munge $EC -> $E0 (used later to ; advance the drive head to the next ; track) 0825- 29 F0 AND #$F0 0827- 85 C8 STA $C8 ; munge $E0 -> $E8 (used later to ; turn off the drive motor) 0829- 09 08 ORA #$08 082B- 85 D6 STA $D6 ; push several addresses to the stack ; (more on this later) 082D- A2 06 LDX #$06 082F- B5 EF LDA $EF,X 0831- 48 PHA 0832- CA DEX 0833- D0 FA BNE $082F ; number of tracks to load (x2) (game- ; specific -- this game uses 9 tracks) 0835- A0 12 LDY #$12 ; loop starts here 083F- 8A TXA ; every other time through this loop, ; we will end up taking this branch 0840- 90 03 BCC $0845 ; X is 0 going into this loop, and it ; never changes, so A is always 0 too. ; So this will push $0000 to the stack ; (to "return" to $0001, which reads a ; track into memory) 0842- 48 PHA 0843- 48 PHA ; There's a "SEC" hidden here (because ; it's opcode $38), but it's only ; executed if we take the branch at ; $0840, which lands at $0845, which is ; in the middle of this instruction. ; Otherwise we execute the compare, ; which clears the carry bit. So the ; carry flip-flops between set and ; clear, so the BCC at $0840 is only ; taken every other time. 0844- C9 38 CMP #$38 ; Push $00B3 to the stack, to "return" ; to $00B4. This routine advances the ; drive head to the next half track. 0846- 48 PHA 0847- A9 B3 LDA #$B3 0849- 48 PHA ; loop until done 084A- 88 DEY 084B- D0 F2 BNE $083F Because of the carry flip-flop, we will push $00B3 to the stack every time through the loop, but we will only push $0000 every other time. The loop runs for twice the number of tracks we want to read, so the stack ends up looking like this: --top-- $00B3 (move drive 1/2 track) $00B3 (move drive another 1/2 track) $0000 (read track into memory) $00B3 \ $00B3 } second group $0000 / $00B3 \ $00B3 } third group $0000 / . . [repeated for each track] . $00B3 \ $00B3 } final group $0000 / $00D4 turn off drive, disable secondary protection $0525 game-specific entry point (pops next two values off the stack, sets up zero page, and pushes actual game entry point) $0000 boot1 checksum value (twice) --bottom-- Boot1 reads the game into memory from tracks $01-$09, but it isn't a loop. It's one routine that reads a track and another routine that advances the drive head. We're essentially unrolling the read loop on the stack, in advance, so that each routine gets called as many times as we need, when we need it. Like dancers in a chorus line, each routine executes then cedes the spotlight. Each seems unaware of the others, but in reality they've all been meticulously choreographed. ~ Chapter 9 6 + 2 Before I can explain the next chunk of code, I need to pause and explain a little bit of theory. As you probably know if you're the sort of person who reads this sort of thing, Apple II floppy disks do not contain the actual data that ends up being loaded into memory. Due to hardware limitations of the original Disk II drive, data on disk must be stored in an intermediate format called "nibbles." Bytes in memory are encoded into nibbles before writing to disk, and nibbles that you read from the disk must be decoded back into bytes. The round trip is lossless but requires some bit wrangling. Decoding nibbles-on-disk into bytes-in- memory is a multi-step process. In "6-and-2 encoding" (used by DOS 3.3, ProDOS, and all ".dsk" image files), there are 64 possible values that you may find in the data field (in the range $96..$FF, but not all of those, because some of them have bit patterns that trip up the drive firmware). We'll call these "raw nibbles." Step 1: read $156 raw nibbles from the data field. These values will range from $96 to $FF, but as mentioned earlier, not all values in that range will appear on disk. Now we have $156 raw nibbles. Step 2: decode each of the raw nibbles into a 6-bit byte between 0 and 63 (%00000000 and %00111111 in binary). $96 is the lowest valid raw nibble, so it gets decoded to 0. $97 is the next valid raw nibble, so it's decoded to 1. $98 and $99 are invalid, so we skip them, and $9A gets decoded to 2. And so on, up to $FF (the highest valid raw nibble), which gets decoded to 63. Now we have $156 6-bit bytes. Step 3: split up each of the first $56 6-bit bytes into pairs of bits. In other words, each 6-bit byte becomes three 2-bit bytes. These 2-bit bytes are merged with the next $100 6-bit bytes to create $100 8-bit bytes. Hence the name, "6-and-2" encoding. The exact process of how the bits are split and merged is... complicated. The first $56 6-bit bytes get split up into 2-bit bytes, but those two bits get swapped (so %01 becomes %10 and vice- versa). The other $100 6-bit bytes each get multiplied by 4 (a.k.a. bit-shifted two places left). This leaves a hole in the lower two bits, which is filled by one of the 2-bit bytes from the first group. A diagram might help. "a" through "x" each represent one bit. ------------- 1 decoded 3 decoded nibble in + nibbles in = 3 bytes first $56 other $100 00abcdef 00ghijkl 00mnopqr | 00stuvwx | split | & shifted swapped left x2 | | V V 000000fe + ghijkl00 = ghijklfe 000000dc + mnopqr00 = mnopqrdc 000000ba + stuvwx00 = stuvwxba ------------- Tada! Four 6-bit bytes 00abcdef 00ghijkl 00mnopqr 00stuvwx become three 8-bit bytes ghijklfe mnopqrdc stuvwxba When DOS 3.3 reads a sector, it reads the first $56 raw nibbles, decoded them into 6-bit bytes, and stashes them in a temporary buffer (at $BC00). Then it reads the other $100 raw nibbles, decodes them into 6-bit bytes, and puts them in another temporary buffer (at $BB00). Only then does DOS 3.3 start combining the bits from each group to create the full 8-bit bytes that will end up in the target page in memory. This is why DOS 3.3 "misses" sectors when it's reading, because it's busy twiddling bits while the disk is still spinning. ~ Chapter 10 Back to 0boot 0boot also uses "6-and-2" encoding. The first $56 nibbles in the data field are still split into pairs of bits that need to be merged with nibbles that won't come until later. But instead of waiting for all $156 raw nibbles to be read from disk, it "interleaves" the nibble reads with the bit twiddling required to merge the first $56 6-bit bytes and the $100 that follow. By the time 0boot gets to the data field checksum, it has already stored all $100 8-bit bytes in their final resting place in memory. This means that 0boot can read all 16 sectors on a track in one revolution of the disk. That's crazy fast. To make it possible to do all the bit twiddling we need to do and not miss nibbles as the disk spins(*), we do some of the work earlier. We multiply each of the 64 possible decoded values by 4 and store those values. (Since this is accomplished by bit shifting and we're doing it before we start reading the disk, this is called the "pre-shift" table.) We also store all possible 2-bit values in a repeating pattern that will make it easy to look them up later. Then, as we're reading from disk (and timing is tight), we can simulate all the bit math we need to do with a series of table lookups. There is just enough time to convert each raw nibble into its final 8-bit byte before reading the next nibble. (*) The disk spins independently of the CPU, and we only have a limited time to read a nibble and do what we're going to do with it before WHOOPS HERE COMES ANOTHER ONE. So time is of the essence. Also, "As The Disk Spins" would make a great name for a retrocomputing-themed soap opera. The first table, at $0200..$02FF, is three columns wide and 64 rows deep. Astute readers will notice that 3 x 64 is not 256. Only three of the columns are used; the fourth (unused) column exists because multiplying by 3 is hard but multiplying by 4 is easy (in base 2 anyway). The three columns correspond to the three pairs of 2-bit values in those first $56 6-bit bytes. Since the values are only 2 bits wide, each column holds one of four different values (%00, %01, %10, or %11). The second table, at $0300..$0369, is the "pre-shift" table. This contains all the possible 6-bit bytes, in order, each multiplied by 4 (a.k.a. shifted to the left two places, so the 6 bits that started in columns 0-5 are now in columns 2-7, and columns 0 and 1 are zeroes). Like this: 00ghijkl --> ghijkl00 Astute readers will notice that there are only 64 possible 6-bit bytes, but this second table is larger than 64 bytes. To make lookups easier, the table has empty slots for each of the invalid raw nibbles. In other words, we don't do any math to decode raw nibbles into 6-bit bytes; we just look them up in this table (offset by $96, since that's the lowest valid raw nibble) and get the required bit shifting for free. addr | raw | decoded 6-bit | pre-shift -----+-----+----------------+---------- $300 | $96 | 0 = %00000000 | %00000000 $301 | $97 | 1 = %00000001 | %00000100 $302 | $98 [invalid raw nibble] $303 | $99 [invalid raw nibble] $304 | $9A | 2 = %00000010 | %00001000 $305 | $9B | 3 = %00000011 | %00001100 $306 | $9C [invalid raw nibble] $307 | $9D | 4 = %00000100 | %00010000 . . . $368 | $FE | 62 = %00111110 | %11111000 $369 | $FF | 63 = %00111111 | %11111100 Each value in this "pre-shift" table also serves as an index into the first table (with all the 2-bit bytes). This wasn't an accident; I mean, that sort of magic doesn't just happen. But the table of 2-bit bytes is arranged in such a way that we take one of the raw nibbles that needs to be decoded and split apart (from the first $56 raw nibbles in the data field), use that raw nibble as an index into the pre- shift table, then use that pre-shifted value as an index into the first table to get the 2-bit value we need. That's a neat trick. ; this loop creates the pre-shift table ; at $300 084D- A2 40 LDX #$40 084F- A4 55 LDY $55 0851- 98 TYA 0852- 0A ASL 0853- 24 55 BIT $55 0855- F0 12 BEQ $0869 0857- 05 55 ORA $55 0859- 49 FF EOR #$FF 085B- 29 7E AND #$7E 085D- B0 0A BCS $0869 085F- 4A LSR 0860- D0 FB BNE $085D 0862- CA DEX 0863- 8A TXA 0864- 0A ASL 0865- 0A ASL 0866- 99 EA 02 STA $02EA,Y 0869- C6 55 DEC $55 086B- D0 E2 BNE $084F And this is the result (".." means the address is uninitialized and unused): 0300- 00 04 .. .. 08 0C .. 10 0308- 14 18 .. .. .. .. .. .. 0310- 1C 20 .. .. .. 24 28 2C 0318- 30 34 .. .. 38 3C 40 44 0320- 48 4C .. 50 54 58 5C 60 0328- 64 68 .. .. .. .. .. .. 0330- .. .. .. .. .. 6C .. 70 0338- 74 78 .. .. .. 7C .. .. 0340- 80 84 .. 88 8C 90 94 98 0348- 9C A0 .. .. .. .. .. A4 0350- A8 AC .. B0 B4 B8 BC C0 0358- C4 C8 .. .. CC D0 D4 D8 0360- DC E0 .. E4 E8 EC F0 F4 0368- F8 FC ; this loop creates the table of 2-bit ; values at $200, magically arranged to ; enable easy lookups later 086D- 46 B7 LSR $B7 086F- 46 B7 LSR $B7 0871- B5 FC LDA $FC,X 0873- 99 FF 01 STA $01FF,Y 0876- E6 AC INC $AC 0878- A5 AC LDA $AC 087A- 25 B7 AND $B7 087C- D0 05 BNE $0883 087E- E8 INX 087F- 8A TXA 0880- 29 03 AND #$03 0882- AA TAX 0883- C8 INY 0884- C8 INY 0885- C8 INY 0886- C8 INY 0887- C0 04 CPY #$04 0889- B0 E6 BCS $0871 088B- C8 INY 088C- C0 04 CPY #$04 088E- 90 DD BCC $086D And this is the result: 0200- 00 00 00 .. 00 00 02 .. 0208- 00 00 01 .. 00 00 03 .. 0210- 00 02 00 .. 00 02 02 .. 0218- 00 02 01 .. 00 02 03 .. 0220- 00 01 00 .. 00 01 02 .. 0228- 00 01 01 .. 00 01 03 .. 0230- 00 03 00 .. 00 03 02 .. 0238- 00 03 01 .. 00 03 03 .. 0240- 02 00 00 .. 02 00 02 .. 0248- 02 00 01 .. 02 00 03 .. 0250- 02 02 00 .. 02 02 02 .. 0258- 02 02 01 .. 02 02 03 .. 0260- 02 01 00 .. 02 01 02 .. 0268- 02 01 01 .. 02 01 03 .. 0270- 02 03 00 .. 02 03 02 .. 0278- 02 03 01 .. 02 03 03 .. 0280- 01 00 00 .. 01 00 02 .. 0288- 01 00 01 .. 01 00 03 .. 0290- 01 02 00 .. 01 02 02 .. 0298- 01 02 01 .. 01 02 03 .. 02A0- 01 01 00 .. 01 01 02 .. 02A8- 01 01 01 .. 01 01 03 .. 02B0- 01 03 00 .. 01 03 02 .. 02B8- 01 03 01 .. 01 03 03 .. 02C0- 03 00 00 .. 03 00 02 .. 02C8- 03 00 01 .. 03 00 03 .. 02D0- 03 02 00 .. 03 02 02 .. 02D8- 03 02 01 .. 03 02 03 .. 02E0- 03 01 00 .. 03 01 02 .. 02E8- 03 01 01 .. 03 01 03 .. 02F0- 03 03 00 .. 03 03 02 .. 02F8- 03 03 01 .. 03 03 03 .. And now for something completely different. The original disk briefly displayed an uninitialized hi-res graphics page (originally at $0801 -- literally the first thing it does on boot). So I want to do the same. It won't be absolutely first thing, but it'll be close. 0890- 2C 54 C0 BIT $C054 0893- 2C 52 C0 BIT $C052 0896- 2C 57 C0 BIT $C057 0899- 2C 50 C0 BIT $C050 089C- 60 RTS [Note to future self: $0890..$08FD is available for game-specific init code, but it can't rely on or disturb zero page in any way. That rules out a lot of built-in ROM routines; be careful. If the game needs no initialization, you can zap this entire range and put an "RTS" at $0890.] Everything else is already lined up on the stack. All that's left to do is "return" and let the stack guide us through the rest of the boot. ~ Chapter 11 0boot boot1 The rest of the boot runs from zero page. It's hard to show you exactly what boot1 will look like, because it relies heavily on self-modifying code. In a standard DOS 3.3 RWTS, the softswitch to read the data latch is "LDA $C08C,X", where X is the boot slot times 16 (to allow disks to boot from any slot). 0boot also supports booting from any slot, but instead of using an index, each fetch instruction is pre- set based on the boot slot. Not only does this free up the X register, it lets us juggle all the registers and put the raw nibble value in whichever one is convenient at the time. (We take full advantage of this freedom.) I've marked each pre-set softswitch with "o_O" to remind you that self-modifying code is awesome. There are several other instances of addresses and constants that get modified while boot1 is running. I've marked these with "/!\" to remind you that self-modifying code is dangerous and you should not try this at home. The first thing popped off the stack is the drive arm move routine at $00B4. It moves the drive exactly one phase (half a track). 00B4- E6 B7 INC $B7 ; This value was set at $00B4 (above). ; It's incremented monotonically, but ; it's ANDed with $03 later, so its ; exact value isn't relevant. 00B6- A0 00 LDY #$00 /!\ ; short wait for PHASEON 00B8- A9 04 LDA #$04 00BA- 20 C0 00 JSR $00C0 ; fall through 00BD- 88 DEY ; longer wait for PHASEOFF 00BE- 69 41 ADC #$41 00C0- 85 CB STA $CB ; calculate the proper stepper motor to ; access 00C2- 98 TYA 00C3- 29 03 AND #$03 00C5- 2A ROL 00C6- AA TAX ; This address was set at $0827, ; based on the boot slot. 00C7- BD E0 C0 LDA $C0E0,X /!\ ; This value was set at $00C0 so that ; PHASEON and PHASEOFF have optimal ; wait times. 00CA- A9 D1 LDA #$D1 /!\ ; wait exactly the right amount of time ; after accessing the proper stepper ; motor 00CC- 4C A8 FC JMP $FCA8 Since the drive arm routine only moves one phase, it was pushed to the stack twice before each track read. Our game is stored on whole tracks; this half- track trickery is only to save a few bytes of code in boot1. The track read routine starts at $0001, because that let us save 1 byte in the boot0 code when we were pushing addresses to the stack. (We could just push $00 twice.) ; sectors-left-to-read-on-this-track ; counter (incremented to $00) 0001- A2 F0 LDX #$F0 0003- 86 00 STX $00 We initialize an array at $00F0 that tracks which sectors we've read from the current track. Astute readers will notice that this part of zero page had real data in it -- some addresses that were pushed to the stack, and some other values that were used to create the 2-bit table at $0200. All true, but all those operations are now complete, and the space from $00F0..$00FF is now available for unrelated uses. The array is in physical sector order, thus the RWTS assumes data is stored in physical sector order on each track. (This is why my MAKE program had to map to physical sector order when writing. This saves 18 bytes: 16 for the table and 2 for the lookup command!) Values are the actual pages in memory where that sector should go, and they get zeroed once the sector is read (so we don't waste time decoding the same sector twice). ; starting address (game-specific; ; this one starts loading at $0400) 0005- A9 04 LDA #$04 /!\ 0007- 95 00 STA $00,X 0009- E6 06 INC $06 000B- E8 INX 000C- D0 F7 BNE $0005 000E- 20 CF 00 JSR $00CF ; subroutine reads a nibble and ; stores it in the accumulator 00CF- AD EC C0 LDA $C0EC o_O 00D2- 10 FB BPL $00CF 00D4- 60 RTS Continuing from $0011... ; first nibble must be $D5 0011- C9 D5 CMP #$D5 0013- D0 F9 BNE $000E ; read second nibble, must be $AA 0015- 20 CF 00 JSR $00CF 0018- C9 AA CMP #$AA 001A- D0 F5 BNE $0011 ; We actually need the Y register to be ; $AA for unrelated reasons later, so ; let's set that now. (We have time, ; and it saves 1 byte!) 001C- A8 TAY ; read the third nibble 001D- 20 CF 00 JSR $00CF ; is it $AD? 0020- 49 AD EOR #$AD ; Yes, which means this is the data ; prologue. Branch forward to start ; reading the data field. 0022- F0 1F BEQ $0043 If that third nibble is not $AD, we assume it's the end of the address prologue. ($96 would be the third nibble of a standard address prologue, but we don't actually check.) We fall through and start decoding the 4-4 encoded values in the address field. 0024- A0 02 LDY #$02 The first time through this loop, we'll read the disk volume number. The second time, we'll read the track number. The third time, we'll read the physical sector number. We don't actually care about the disk volume or the track number, and once we get the sector number, we don't verify the address field checksum. 0026- 20 CF 00 JSR $00CF 0029- 2A ROL 002A- 85 AC STA $AC 002C- 20 CF 00 JSR $00CF 002F- 25 AC AND $AC 0031- 88 DEY 0032- 10 F2 BPL $0026 ; store the physical sector number ; (will re-use later) 0034- 85 AC STA $AC ; use physical sector number as an ; index into the sector address array 0036- A8 TAY ; get the target page (where we want to ; store this sector in memory) 0037- B6 F0 LDX $F0,Y ; store the target page in several ; places throughout the following code 0039- 86 9B STX $9B 003B- CA DEX 003C- 86 6B STX $6B 003E- 86 83 STX $83 0040- E8 INX ; This is an unconditional branch, ; because the ROL at $0029 will always ; set the carry. We're done processing ; the address field, so we need to loop ; back and wait for the data prologue. 0041- B0 CB BCS $000E ; execution continues here (from $0022) ; after matching the data prologue 0043- E0 00 CPX #$00 ; If X is still $00, it means we found ; a data prologue before we found an ; address prologue. In that case, we ; have to skip this sector, because we ; don't know which sector it is and we ; wouldn't know where to put it. 0045- F0 C7 BEQ $000E Nibble loop #1 reads nibbles $00..$55, looks up the corresponding offset in the preshift table at $0300, and stores that offset in the temporary buffer at $036A. ; initialize rolling checksum to $00 0047- 85 55 STA $55 0049- AE EC C0 LDX $C0EC o_O 004C- 10 FB BPL $0049 ; The nibble value is in the X register ; now. The lowest possible nibble value ; is $96 and the highest is $FF. To ; look up the offset in the table at ; $0300, we need to subtract $96 from ; $0300 and add X. 004E- BD 6A 02 LDA $026A,X ; Now the accumulator has the offset ; into the table of individual 2-bit ; combinations ($0200..$02FF). Store ; that offset in the temporary buffer ; at $036A, in the order we read the ; nibbles. But the Y register started ; counting at $AA, so we need to ; subtract $AA from $036A and add Y. 0051- 99 C0 02 STA $02C0,Y ; The EOR value is set at $0047 ; each time through loop #1. 0054- 49 00 EOR #$00 /!\ 0056- C8 INY 0057- D0 EE BNE $0047 Here endeth nibble loop #1. Nibble loop #2 reads nibbles $56..$AB, combines them with bits 0-1 of the appropriate nibble from the first $56, and stores them in bytes $00..$55 of the target page in memory. 0059- A0 AA LDY #$AA 005B- AE EC C0 LDX $C0EC o_O 005E- 10 FB BPL $005B 0060- 5D 6A 02 EOR $026A,X 0063- BE C0 02 LDX $02C0,Y 0066- 5D 02 02 EOR $0202,X ; This address was set at $003C ; based on the target page (minus 1 ; so we can add Y from $AA..$FF). 0069- 99 56 D1 STA $D156,Y /!\ 006C- C8 INY 006D- D0 EC BNE $005B Here endeth nibble loop #2. Nibble loop #3 reads nibbles $AC..$101, combines them with bits 2-3 of the appropriate nibble from the first $56, and stores them in bytes $56..$AB of the target page in memory. 006F- 29 FC AND #$FC 0071- A0 AA LDY #$AA 0073- AE EC C0 LDX $C0EC o_O 0076- 10 FB BPL $0073 0078- 5D 6A 02 EOR $026A,X 007B- BE C0 02 LDX $02C0,Y 007E- 5D 01 02 EOR $0201,X ; This address was set at $003E ; based on the target page (minus 1 ; so we can add Y from $AA..$FF). 0081- 99 AC D1 STA $D1AC,Y /!\ 0084- C8 INY 0085- D0 EC BNE $0073 Here endeth nibble loop #3. Loop #4 reads nibbles $102..$155, combines them with bits 4-5 of the appropriate nibble from the first $56, and stores them in bytes $AC..$FF of the target page in memory. 0087- 29 FC AND #$FC 0089- A2 AC LDX #$AC 008B- AC EC C0 LDY $C0EC o_O 008E- 10 FB BPL $008B 0090- 59 6A 02 EOR $026A,Y 0093- BC BE 02 LDY $02BE,X 0096- 59 00 02 EOR $0200,Y ; This address was set at $0039 ; based on the target page. 0099- 9D 00 D1 STA $D100,X /!\ 009C- E8 INX 009D- D0 EC BNE $008B Here endeth nibble loop #4. ; Finally, get the last nibble, ; which is the checksum of all ; the previous nibbles. 009F- 29 FC AND #$FC 00A1- AC EC C0 LDY $C0EC o_O 00A4- 10 FB BPL $00A1 00A6- 59 6A 02 EOR $026A,Y ; if checksum fails, start over 00A9- D0 96 BNE $0041 ; This was set to the physical ; sector number (at $0034), so ; this is a index into the 16- ; byte array at $00F0. 00AB- A0 C0 LDY #$C0 /!\ ; store $00 at this index in the sector ; array to indicate that we've read ; this sector 00AD- 96 F0 STX $F0,Y ; are we done yet? 00AF- E6 00 INC $00 ; nope, loop back to read more sectors 00B1- D0 8E BNE $0041 ; And that's all she read. 00B3- 60 RTS 0boot's track read routine is done when $0000 hits $00, which is astonishingly beautiful. Like, "now I know God" level of beauty. And so it goes: we pop another address off the stack, move the drive arm, read another track, and eventually pop off the final routine at $00D5: ; turn off drive motor 00D5- AD E8 C0 LDA $C0E8 /!\ ; disable secondary protection 00D8- A9 60 LDA #$60 00DA- 8D C0 8E STA $8EC0 00DD- 60 RTS The "RTS" at $DD will pop the next address off the stack ($05/$25) and continue at $0526. As we saw earlier, that routine immediately pops the next two values off the stack and uses them to set up $00/$01, $36/$37, $0100, and jump to the game's entry point ($8000). But we don't need to do any of that ourselves. We just need to prepare the stack, then the original code can do what it's designed to do. Minus the protect-y bits, of course. Quod erat liberandum. ~ Changelog 2020-06-24 - typo in the 6-and-2 encoding diagram [thanks Andrew R.] 2015-12-12 - initial release --------------------------------------- A 4am crack No. 516 ------------------EOF------------------