Uptime in ZX BASIC

Question

How can I obtain the uptime of a Sinclair ZX Spectrum machine (ZX48, ZX128, or Pentagon clone) via their BASIC interpreter? Specifically, the same value used by RANDOMIZE 0 to seed its PRNG. I would assume that this uptime would be the number of cycles the MCU has gone through since boot and so would increment at 3.5 MHz (or some fraction thereof). If it is not possible to obtain through the BASIC interpreter, how would I go about getting it by writing code directly to memory, e.g. with DATA?

The reason I am wondering is because I would like to generate a sequence of random numbers less predictable than that generated by RND. To do that, I need an environmental source of entropy. Inter-keystroke timing is the perfect place to get that entropy (as the ZX has no HWRNG), but only if a high-resolution timer can be sampled at each keystroke. If this is an X/Y question, please let me know.

Answer 1

According to http://www.worldofspectrum.org/ZXBasicManual/zxmanchap25.html, addresses 23672-23674 contain a 24 bit count of 50Hz frame ticks in the UK. I wrote a quick program to print the values, which do indeed seem to start at zero on startup and increment at the right rate, and thus serve as an uptime counter. A simple bit of maths indicates that the counter wraps round roughly once every four days.

Answer 2

I would assume that this uptime would be the number of cycles the MCU has gone through since boot and so would increment at 3.5 MHz (or some fraction thereof).

As already mentioned by pndc, the 24 bit counter FRAMES at 5C78h increments (*1) with every frame (*2) displayed and is zeroed at startup. Thus, an overflow happens only after more than 4 days of gameplay :))

The reason I am wondering is because I would like to generate a sequence of random numbers less predictable than that generated by RAND.

FRAMES, or better its lower 16 bits, are already used to initialize the PRNG when RANDOMIZE 0is called, as Tofro pointed out. So there is some randomness, but then again, it's strictly based on the program run, unless the program includes many waiting times for asynchronous external events (like keyboard wait).

To do that, I need an environmental source of entropy. Inter-keystroke timing is the perfect place to get that entropy (as the ZX has no HWRNG), but only if a high-resolution timer can be sampled at each keystroke.

Hmm, there is kind of an high precision timer in every Z80: the refresh (R) Register. It gets incremented in every T3 and T4 clocks of an M1 cycle. That's once per instruction (*3). Only the lower 7 bit are valid, so you may want to collect them if you need more bits. Just make sure there's something un-/less predictable in between.

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*1 - Which is an interesting deviation from the ZX81, where a similar counter was decremented

*2 - That's 50 times on European models and 60 on NTSC-Spectrums (sold via mail order in the US) and US-TS2068

*3 - It's a bit more complex, so see this fine Stackoverflow answer for details.

Answer 3

To do that, you simply initialise the random number generator with 0, like

RANDOMIZE 0

For this specific argument, according to the ROM disassembly, the ZX Spectrum BASIC will pick the value from the lower two bytes of the System variable FRAMES, which is the number of 50Hz interrupts since startup.

In case you want the seed value for your own PRNG, simply PEEK that system variable (05C76H), as pointed out in other answers (But be aware the Spectrum's PRNG isn't actually that bad for an 80ies 8-bit computer and you might be having a hard time writing a better one yourself. The PNRG is actually quite cleverly made without a lot of code because it uses the Arithmetic interpreter of the Spectrum.) You could also RANDOMIZE 0 to make the initial VALUE of RND dependant on the switched-on time and then feed your own, better implementation of a PRNG with the first value obtained by RND.

Answer 4

Could perform some in(port) commands instead - some ports did not have anything to define the read value, so could return random results. This was very version specific and additional hardware specific too. Not sure what emulators would do.

You could look at the tape port - just play (loud) music at it.

You would watch the keyboard ports (0xfe) directly to see which keys were pressed - loop counter detecting a change is more like your inter-keystroke timing.

10 LET A=0
20 LET B=0
30 LET C=IN (254)
40 IF (B<>C) THEN GOSUB 1000
50 LET A=A+1
60 GOTO 30
1000 PRINT A
1010 LET B=C
1020 RETURN

BASIC is slow; probably need to do that in machine code.

Answer 5

If RANDOMIZE 0 isn't sufficiently random for your purposes, you could invoke it several times at unpredictable intervals (e.g. using keypress timings as you suggest), then combine the results. You probably want to keep values within the range 0-65535, so that the Spectrum uses exact integers rather than floating-point numbers.

Off-hand (and having not written any ZX Basic for 30 years!), I'd do the combining of partial result r with a new random number something like

LET r = r % 255 * 257 + RND % 256

This allows us to multiply by 257 (which I think is prime), without overflowing the integer range.