Load routines
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5D17: 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 5D17.
Entering the code here from BASIC means that C=0x17 (the LSB of 5D17); this value is used later on in the routine at 802F.
5D17 DI Disable interrupts
5D18 LD B,$31
5D1A LD DE,$FFCF
5D1D CALL $5DEE
And so begins the misdirection.
5DEE POP HL HL=5D20
5DEF ADD HL,DE HL=5CEF
5DF0 PUSH HL
5DF1 LD DE,$0007
5DF4 ADD HL,DE HL=5CF6
5DF5 RET Jump to 5CEF
Undo some code obfuscation before continuing.
5CEF LD A,(HL) Do some XOR-ing on the 49 bytes from 5CF6 to 5D26
5CF0 XOR H
5CF1 XOR L
5CF2 LD (HL),A
5CF3 INC HL
5CF4 DJNZ $5CEF
After the XOR-ing:
5CF6 EXX
5CF7 EX (SP),HL HL'=2D2B
5CF8 LD A,H A=0x2D
5CF9 AND L A=0x29
5CFA ADD A,L A=0x54
5CFB ADD A,L A=0x7F
5CFC LD H,A H'=0x7F
5CFD ADD A,$4E A=0xCD
5CFF LD L,A HL'=7FCD
5D00 EXX
5D01 LD HL,$5BFF INK 1: PAPER 1
5D04 LD (HL),$09
5D06 DEC HL
5D07 BIT 3,H
5D09 JR NZ,$5D04
5D0B LD (HL),$00 Clear the display file
5D0D DEC HL
5D0E BIT 6,H
5D10 JR NZ,$5D0B
5D12 INC E DE=0009
5D13 INC E
5D14 CALL $5DEE
More misdirection.
5DEE POP HL HL=5D17
5DEF ADD HL,DE HL=5D20
5DF0 PUSH HL
5DF1 LD DE,$0007
5DF4 ADD HL,DE HL=5D27
5DF5 RET Jump to 5D20
Undo some more code obfuscation before continuing.
5D20 LD A,(HL) Do some XOR-ing and RLC-ing on the 217 bytes from 5D27 to 5DFF
5D21 XOR E
5D22 RLCA
5D23 LD (HL),A
5D24 INC L
5D25 JR NZ,$5D20
After the XOR-ing and RLC-ing:
5D27 PUSH HL HL=5D00
5D28 EXX
5D29 POP DE DE'=5D00
5D2A EX DE,HL DE'=7FCD
5D2B LD BC,$00FF Copy 255 bytes from 5D00-5DFE to 7FCD-80CB
5D2E LDIR
5D30 LD E,A E'=0x2F
5D31 EX DE,HL HL'=802F
5D32 JP (HL) Jump to 802F
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