Back to Skool: Routine at 5D17

5D17: After the program 'bak2skool' has loaded, this is where it all starts...

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

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