Yie Ar Kung Fu - example self-decrypting loader and returning to Basic II's RUN routine at &BD13, by pushing that address onto the stack so RTS will go to it at the end and start the Basic program.
0DE8 LDY #0 \
0DEA STY &70 \
0DEC LDA &0E00,Y \ decrypt PAGE &E
0DEF EOR #&FF \ by EORing bytes with &FF
0DF1 DEC &0DF5 \ and a byte going &CB down to &CC
0DF4 EOR #&CC
0DF6 EOR &70 \ and a byte going 0 up to &FF
0DF8 INC &70
0DFA STA &0E00,Y
0DFD INY
0DFE BNE &0DEC
The next bit can be 'zapped' by typing the following, which is a Basic
version of the above code:
File YIE3Z now contains the decrypted code. By the time we get here the code is decrypted
0E00 LDA #3 \Disable Escape and clear memory on Break
0E02 STA &0258 \*FX200,3 equivalent
0E05 LDA #&B3 \&56B13
0E07 STA &12 \is the TOP of the Basic control program
0E09 LDA #&56
0E0B STA &13
0E0D LDA #&BD \&BD13 is the Basic II rom RUN routine
0E0F PHA \so when we RTS at the end of this
0E10 LDA #&13 \it will be like typing RUN to start
0E12 PHA \the game
0E13 LDA #&CC \value used later for decryption
0E15 PHA \pushed on stack
0E16 LDX #&16 \fill &E00-&E16 with &16 to corrupt
0E18 TXA \program to prying eyes
0E19 STA &0E00,X
0E1C DEX
0E1D BPL &0E19
0E1F LDA #140 \*FX140,12
0E21 LDX #12 \*TAPE equivalent
0E23 JSR OSbyte
0E26 LDX #&4D \string at &E4D
0E28 LDY #&0E
0E2A JSR OScli \do *LOAD YIE3??? 5200
0E2D LDY #0
0E2F LDX #&47 \prepare to decrypt &47 pages
0E31 PLA \retrieve value stacked earlier
0E32 STA &0E39
0E35 LDA &1100,Y
0E38 EOR #3 \this is EOR #&CC see line &E32
0E3A STA &1100,Y \in Basic this would be
0E3D INY \FORX%=&1100TO&47FF:?X%=?X% EOR&CC:NEXT
0E3E BNE &1E35
0E40 INC &0E37 \increment hi byte of source address
0E43 EE 3C 0E INC &0E3C \increment hi byte of destination address
0E46 DEX
0E47 BNE &0E35 \loop till all pages done
0E49 STY &0E39 \so instruction at &E38 is EOR #0
0E4C RTS \return to &BD13 - Basic RUN
The next bit is the OSCLI string - *L. YIE3??? 5200
16th April 2010