CPU 1 \ Switch to 65C02 assembly, because although this \ routine forms part of the code that runs on the 6502 \ CPU of the BBC Micro I/O processor, the do65C02 \ routine gets transmitted across the Tube to the \ parasite, and it contains some 65C02 code .do65C02 .whiz \ When the following code is run as part of the S% \ routine in the parasite, it is entered with the \ following set: \ \ (1 0) = SC(1 0) = G% \ \ (3 2) = F% - 1 \ \ X = SC \ \ We can access the address in (1 0) via indirect \ addressing, as in LDA (0), which is the same as \ LDA (&0000), and loads the byte at the 16-bit address \ in locations &0000 and &0001, or (1 0). In the same \ way, LDA (2) loads the byte at the address in (3 2) LDA (0) \ Swap the bytes at the addresses (1 0) and (3 2), so PHA \ this starts by swapping G% and F%-1, and moves on to LDA (2) \ G%+1 and F%-2, then G%+2 and F%-3, and so on, until STA (0) \ (1 0) and (3 2) meet in the middle PLA STA (2) \NOP \NOP \NOP \NOP INC 0 \ Increment the low byte of (1 0) to move it on to the \ next byte BNE P%+4 \ If the low byte has not wrapped round to zero, skip \ the following instruction INC 1 \ Increment the high byte of (1 0) to move it on to the \ first byte of the next page LDA 2 \ Set A to the low byte of (3 2) BNE P%+4 \ If the low byte is not zero, skip the following \ instruction DEC 3 \ Decrement the high byte of (3 2) to move it on to the \ last byte of the previous page DEC 2 \ Decrement the low byte of (3 2) to move it on to the \ previous byte DEA \ Decrement A, which we set to the low byte of (3 2) \ above, so A now equals (2), the new low byte of (3 2) CMP 0 \ If A < low byte of (1 0), i.e. low byte of (3 2) < low \ byte of (1 0), then clear the C flag, else set it LDA 3 \ Set A = high byte of (3 2) - high byte of (1 0) SBC 1 \ - 1 if low byte of (3 2) < low byte of (1 0) \ \ so this subtraction will underflow and clear the C \ flag when the high bytes of (3 2) and (1 0) are equal \ and low byte of (3 2) < low byte of (1 0), which will \ happen when the two endpoints cross over in the \ middle BCS whiz \ If the C flag is set then (1 0) < (3 2), so loop back \ to reverse more bytes, as we haven't yet crossed over \ in the middle JMP (0,X) \ Jump to (0+X), which is the same as (X). We set X to \ SC in S% before entering this routine, so this jumps \ to the address in SC(1 0), which contains G%... so \ this jumps to G% to start the game .end65C02 protlen = end65C02 - do65C02 CPU 0 \ Switch back to normal 6502 asemblyName: do65C02 [View in context] Type: Subroutine Category: Copy protection Summary: Reverse the order of all bytes between the addresses in (1 0) and (3 2) and start the game
This routine is copied into the parasite's memory when it sends an OSWORD 249 command to the I/O processor. The code is copied by returning it in the OSWORD parameter block. It ends up at location prtblock+2 in the S% routine. When run, this routine reverses the order of all bytes between the address in (1 0) and the address in (3 2). It starts by swapping the bytes at each end of the memory block, and moves towards the centre of the block, swapping as it goes until the two ends meet in the middle, where it stops. In the original source, the memory is reversed by the first call to routine V in the Big Code File, though in the BeebAsm version this is populated by elite-checksum.py. The original 6502 assembly language version of the V routine can be found in the elite-checksum.asm file.
Label whiz is local to this routine