Comment by jgrahamc

Comment by jgrahamc 2 days ago

48 replies

View on Hacker News

In my 6502 hacking days, the presence of an exclusive OR was a sure-fire indicator you’d either found the encryption part of the code, or some kind of sprite routine.

Yeah, sadly the 6502 didn't allow you to do EOR A; while the Z80 did allow XOR A. If I remember correctly XOR A was AF and LD A, 0 was 3E 01[1]. So saved a whole byte! And I think the XOR was 3 clock cycles fast than the LD. So less space taken up by the instruction and faster.

I have a very distinct memory in my first job (writing x86 assembly) of the CEO walking up behind my desk and pointing out that I'd done MOV AX, 0 when I could have done XOR AX, AX.

[1] 3E 00

wavemode 2 days ago

> CEO walking up behind my desk and pointing out that I'd done MOV AX, 0 when I could have done XOR AX, AX

Now that's what I call micromanagement.

(sorry couldn't resist)

Reply View 17 replies
  • xigoi 2 days ago

    The real joke is that a CEO had actual technical knowledge instead of just being there for decoration.

    Reply View 0 replies
  • jgrahamc 2 days ago

    He was right though. We were memory and cycle constrained and I'd wasted both!

    Reply View 0 replies
  • mkornaukhov 2 days ago

    Similarly, the CEO couldn't resist the outstanding optimization of memory and execution speed!

    Reply View 12 replies
    • 6510 2 days ago

      [flagged]

      Reply View 11 replies
      • jgrahamc 2 days ago

        I am sad you don't believe this story. The CEO was very technical and this is exactly the sort of thing he would spot.

        Reply View 9 replies
      • eru 2 days ago

        CEO doesn't need to mean some big boss. If you have a three person startup, the CEO might just be your co-founding buddy.

        Reply View 0 replies
  • crest 2 days ago

    I had to pad the code for alignment reasons. ;-)

    Reply View 0 replies
stevefan1999 2 days ago

> In my 6502 hacking days, the presence of an exclusive OR was a sure-fire indicator you’d either found the encryption part of the code, or some kind of sprite routine.

Correct. Most ciphers of that era should be Feistel cipher in the likes of DES/3DES, or even RC4 uses XOR too. Later AES/Rijndael, CRC and ECC (Elliptic Curve Cryptography) also make heavy use of XOR but in finite field terms which is based on modular arithmetic over GF(2), that effectively reduces to XOR (while in theory should be mod 2).

Reply View 9 replies
  • OhMeadhbh 2 days ago

    I was going to say "but RC4 and AES were published well after the 6502's heyday," but NESes were completely rocking it in '87 (and I'm told 65XX cores were used as the basis for several hard drive controllers of the era.) Alas, the closest I ever came to encryption on a (less than 32-bit system) was lucifer on an IBM channel controller in the forever-ago and debugging RC5 on an 8085.

    Reply View 5 replies
    • kjs3 2 days ago

      I'm told 65XX cores were used as the basis for several hard drive controllers of the era

      Western Design Center is still (apparently) making a profit at least in part licensing 6502 core IP for embedded stuff. There's probably a 6502 buried and unrecognized in all sorts of low-cost control applications laying around you.

      RC5 on an 8085

      Oof. Well played.

      Reply View 4 replies
      • PaulHoule 2 days ago

        I dunno. The 6502 has been a $2 part for a long time but needs RAM and some glue logic, for a similar price you can get an AVR-8 [1] or ESP-32 [2] and get some RAM and GPIO.

        [1] faster, more registers than the IBM 360, << 64k RAM

        [2] much faster, 32bit, >> 64k RAM

        Reply View 3 replies
  • ASalazarMX 2 days ago

    Reading cryptography was that advanced at that time, I'm even more surprised that the venerable Norton Utilities for MS-DOS required a password, that was simply XORed with some constant and embedded in the executables. If the reserved space was zeroes, it considered it a fresh install and demanded a new password.

    If it had been properly encrypted my young cracker self would have had no opportunity.

    Reply View 0 replies
  • stevefan1999 13 hours ago

    Self-correction: It is GF(2^8) and not GF(2), but GF(2^8) primitive operations (such as carryless multiplication) can be reduced into a bunch of table lookups and/or GF(2) operations, which is how to AES crypto accelerators are being done in hardware.

    Reply View 0 replies
  • Sesse__ a day ago

    Well, running in CTR mode is really common now, and that ends up XORing the generated keystream into the plaintext… (CTR mode is essentially converting block ciphers into stream ciphers, if you want to see it that way.)

    Reply View 0 replies
vanderZwan 2 days ago

Hah, we commented on the exact same paragraph within a minute of each other! My memory agrees with your memory, although I think that should be 3E 00. Let me look that up:

https://jnz.dk/z80/ld_r_n.html

https://jnz.dk/z80/xor_r.html

Yep, if I'm reading this right that's 3E 00, since the second byte is the immediate value.

One difference between XOR and LD is that LD A, 0 does not affect flags, which sometimes mattered.

Reply View 5 replies
  • sfink 2 days ago

    What is this "LD A, 0" syntax? Is it a z80 thing?

    One of the random things burned into my memory for 6502 assembly is that LDA is $A9. I never separated the instruction from the register; it's not like they were general purpose. But that might be because I learned programming from the 2 books that came with my C64, a BASIC manual and a machine code reference manual, and that's how they did it.

    I learned assembly programming by reading through the list of supported instructions. That, and typing in games from Compute's Gazette and manually disassembling the DATA instructions to understand how they worked. Oh, and the zero-page reference.

    Good times.

    Reply View 3 replies
    • Narishma 2 days ago

      > One of the random things burned into my memory for 6502 assembly is that LDA is $A9. I never separated the instruction from the register; it's not like they were general purpose.

      You had LDA and LDX and LDY as separate instructions while the Z80 assembler had a single LD instruction with different operands. It's the same thing really.

      Reply View 1 reply
      • sfink 2 days ago

        Right, though the LD? and ST? instructions were kind of exceptions. You could only do arithmetic and stack and bitwise ops (and, or, eor, shift, rotate) with A, never X nor Y. Increment and decrement were X/Y only. You couldn't even add two registers together without stashing one in memory.

        Reply View 0 replies
    • vanderZwan a day ago

      > What is this "LD A, 0" syntax? Is it a z80 thing?

      Well, I never wrote any 6502 so I can't compare, but yes, you could load immediate values into any register except the flag register on the Z80. Was that not a thing on the 6502?

      Reply View 0 replies
  • jgrahamc 2 days ago

    You're right. Of course, it's 3E 00. Not sure how I remembered 3E 01. My only excuse is that it was 40 years ago!

    Reply View 0 replies
anonzzzies 2 days ago

3E 00 : I was on MSX and never had an assembler when you so I only remember the Hex, never actually knew the instructions; I wrote programs/games by data 3E,00,CD,etc without comments saying LD A as I never knew those at the time.

Reply View 8 replies
  • unnah 2 days ago

    Umm... how did you manage to learn those hex codes? You just read a lot of machine code and it started to make sense?

    Reply View 7 replies
    • jgrahamc 2 days ago

      I started out writing machine code without an assembler and so had to hand assemble a lot of stuff. After a while you end up just knowing the common codes and can write your program directly. This was also useful because it was possible to write or modify programs directly through an interface sometimes called a "front panel" where you could change individual bytes in memory.

      Back in 1985 I did some hand-coding like this because I didn't have access to an assembler: https://blog.jgc.org/2013/04/how-i-coded-in-1985.html and I typed the whole program in through the keypad.

      Reply View 1 reply
      • stevekemp 2 days ago

        Same here. On/For the ZX Spectrum, looking up the hex-codes in the back of the orange book. At least it was spiral-bound to make it easier.

        Later still I'd be patching binaries to ensure their serial-checks passed, on Intel.

        Reply View 0 replies
    • af78 2 days ago

      I had a similar experience of writing machine code for Z80-based computers (Amstrad CPC) in the 90's, as a teenager. I didn't have an assembler so I manually converted mnemonics to hex. I still remember a few opcodes: CD for CALL, C9 for RET, 01 for LD BC, 21 for LD HL... Needless to say, the process was tedious and error-prone. Calculating relative jumps was a pain. So was keeping track of offsets and addresses of variables and jump targets. I tended to insert nops to avoid having to recalculate everything in case I needed to modify some code... I can't say I miss these times.

      I'm quite sure none of my friends knew any CPU opcode; however, people usually remembered a few phone numbers.

      Reply View 0 replies
    • senderista 2 days ago

      It wasn't unusual in the 80s to type in machine code listings to a PC; I remember doing this as an 8-year-old from magazines, but I didn't understand any of the stuff I was typing in.

      Reply View 0 replies
    • anonzzzies 2 days ago

      Typing from mags, getting interested in how the magic works by learning to use a hex monitor and trying out things. I was a kid so time enough.

      I didn't know you could do it differently for years after I started.

      Reply View 0 replies
    • amirhirsch 2 days ago

      I implemented a PDP-11 in 2007-10 and I can still read PDP-11 Octal

      Reply View 0 replies
mmphosis 2 days ago

Try to keep the value 0 in the Y register.

  echo tya|asm|mondump -r|6502
                                A=AA X=00 Y=00 S=00 P=22 PC=0300  0
  0300- 98        TYA           A=00 X=00 Y=00 S=00 P=22 PC=0301  2
Reply View 2 replies
  • brucehoult 2 days ago

    That's 1 byte smaller than `LDA #0`, but not faster. And you don't have enough registers to waste them -- being able to do `STZ` and the `(zp)` addressing mode without having to keep 0 in Z or Y were small but soooo convenient things in the 65C02.

    Reply View 1 reply
    • snvzz a day ago

      You might like the PC Engine, a game console based on the 65C02*.

      *Actually a custom chip also containing some peripherals.

      Reply View 0 replies