Load routines
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23831: After the program 'bak2skool' has loaded, this is where it all starts...
loader This is the initial loader program for Back to Skool (as it appears after doing 'POKE 23762,0' to change the INK for the BASIC listing from white to black). Don't be fooled by the 'RANDOMIZE USR 23794': the floating point number stored underneath is actually 23831.
Entering the code here from BASIC means that C=23 (the LSB of 23831); this value is used later on in the routine at 32815.
23831 DI Disable interrupts
23832 LD B,49
23834 LD DE,65487
23837 CALL 24046
And so begins the misdirection.
24046 POP HL HL=23840
24047 ADD HL,DE HL=23791
24048 PUSH HL
24049 LD DE,7
24052 ADD HL,DE HL=23798
24053 RET Jump to 23791
Undo some code obfuscation before continuing.
23791 LD A,(HL) Do some XOR-ing on the 49 bytes from 23798 to 23846
23792 XOR H
23793 XOR L
23794 LD (HL),A
23795 INC HL
23796 DJNZ 23791
After the XOR-ing:
23798 EXX
23799 EX (SP),HL HL'=11563
23800 LD A,H A=45
23801 AND L A=41
23802 ADD A,L A=84
23803 ADD A,L A=127
23804 LD H,A H'=127
23805 ADD A,78 A=205
23807 LD L,A HL'=32717
23808 EXX
23809 LD HL,23551 INK 1: PAPER 1
23812 LD (HL),9
23814 DEC HL
23815 BIT 3,H
23817 JR NZ,23812
23819 LD (HL),0 Clear the display file
23821 DEC HL
23822 BIT 6,H
23824 JR NZ,23819
23826 INC E DE=9
23827 INC E
23828 CALL 24046
More misdirection.
24046 POP HL HL=23831
24047 ADD HL,DE HL=23840
24048 PUSH HL
24049 LD DE,7
24052 ADD HL,DE HL=23847
24053 RET Jump to 23840
Undo some more code obfuscation before continuing.
23840 LD A,(HL) Do some XOR-ing and RLC-ing on the 217 bytes from 23847 to 24063
23841 XOR E
23842 RLCA
23843 LD (HL),A
23844 INC L
23845 JR NZ,23840
After the XOR-ing and RLC-ing:
23847 PUSH HL HL=23808
23848 EXX
23849 POP DE DE'=23808
23850 EX DE,HL DE'=32717
23851 LD BC,255 Copy 255 bytes from 23808-24062 to 32717-32971
23854 LDIR
23856 LD E,A E'=47
23857 EX DE,HL HL'=32815
23858 JP (HL) Jump to 32815
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