#2byte to 1 byte evom code
#2byte to 1 byte evom plus

2Byte Games makes no warranties about the accuracy or completeness of any information contained on this web site.ĢByte Games, including its directors, affiliates, officers, employees, agents, contractors, successors and assigns, will not accept any liability for any loss, damage or other injury resulting from its use. It may be in use in homebrew-era games (2010 and later).The information provided on this 2Byte Games web site has been compiled for your convenience. But because of the cost of copying a mask ROM, this sort of watermarking wasn't actually used in games during the original commercial era of the Famicom and NES (1983 to 1996). This can help identify exactly which copy of a game was leaked to the warez scene.Īs with clockslide, watermarking can also be done without unofficial instructions. And now that NES games are manufactured with flash memory instead of mask ROM, each cartridge can have a slightly different pattern of NOPs. But it adds even more entropy to use unofficial NOPs ( $1A, $3A, $5A, $7A, $DA, or $FA), two-byte NOPs ( $80 ii, $82 ii, $89 ii, $C2 ii, $E2 ii), or two-byte NOPs that read the zero page ( $04 dd, $44 dd, or $64 dd).

#2byte to 1 byte evom code

This can identify authorship of a piece of code in a way similar to trap streets. WatermarkingĪs LOIS 16192 mentioned, the official NOP instruction ( $EA) can be inserted at random places in a particular subroutine that isn't an inner loop. It's even more important on the Atari 2600, where the whole screen is a raster effect.ĬMP has a side effect of destroying most of the processor status flags, but unofficial instructions that skip one byte can be used to preserve them.įor example, replace $C9 ( CMP) with $89 or $80, which skips one immediate byte, and replace $C5 with $04, $44, or $64, which reads a byte from zero page and ignores it.

Disassemble one byte in and you get CMP #$C9 CMP #$C5 NOP (5 bytes, 6 cycles).Ī calculated start address into a clockslide can be used with indirect jumps ( JMP (aaaa) or LDA highbyte PHA LDA lowbyte PHA RTS) to precisely control timing, such as when playing PCM audio or sending video register changes to the PPU in a raster effect.

Disassemble from the start and you get CMP #$C9 CMP #$C9 CMP $00EA (6 bytes, 7 cycles).With official instructions, one can construct a clockslide from CMP instructions.

#2byte to 1 byte evom plus

ClockslideĪ clockslide is a is a sequence of instructions that wastes a small constant amount of cycles plus one cycle per executed byte, no matter whether it's entered on an odd or even address. One hypothesis is that a programmer working on both NES projects and projects for some 65C02-based system forgot that the original 6502 lacked BIT #ii, but because the instruction does so little anyway, the programmer didn't notice any difference. On the 65C02, this instruction is changed to BIT #ii, which almost behaves as a two-byte NOP. Based on adjacent instructions in the opcode matrix, especially LDA #ii ( $A9 ii), it would have been STA #ii, a store to an immediate value, which makes no sense.

The instruction $89 on the 6502 is a two-byte NOP.