What are tacts in the context of ZX Spectrum systems?

Question

I've heard the term tacts when referring to some hardware capability, e.g. "tacts on one frame" or "tacts per second". I tried Googling this but surprisingly did not find anything relevant. What is the meaning of this term when used in the aforementioned contexts?

Answer 1

The word "такт" is routinely used in Russian to denote exactly "clock cycle" in digital electronics.

It always was and still is heavily used in the Russian ZX scene in the same meaning (e.g. counting clock cycles of Z80 CPU, etc.) and sometimes was (and is) leaking to the English texts written by native Russian speakers.

So "tact" could be recognized as a semantic loan from Russian.

Answer 2

Clock ticks as seen by the processor - e.g. a NOP instruction takes 4 tacts, or a frame takes 69888 tacts (on a 48K Spectrum). It's a fairly common usage of the term within computing circles (not just the Spectrum), being borrowed from the musical meaning.

Answer 3

1. Tact [T]

   This term is equivalent for CPU clock period and it is the smallest time chunk in CPU based computer timings. One level higher are the MC machine cycles (each MC is 1 or more tacts depends on MC type and CPU architecture). So if you see something like 10T that means it is time duration of 10 periods of CPU clock.

   This way of timing notation is independent on CPU frequency so you can compute duration of code universally and apply CPU clock on the final time ... It is used mostly for individual instruction timings. Some instructions have 2 times like 8/10T which means instruction can run in either 8T or 10T depending on something (like state of flags, condition, etc).

   Here some example instructions timing for Z80:

   opc      T0 T1 MC1   MC2   MC3   MC4   MC5   MC6   MC7   mnemonic
   
   B8       04 00 M1R 4 ... 0 ... 0 ... 0 ... 0 ... 0 ... 0 CP A,B
   B9       04 00 M1R 4 ... 0 ... 0 ... 0 ... 0 ... 0 ... 0 CP A,C
   BA       04 00 M1R 4 ... 0 ... 0 ... 0 ... 0 ... 0 ... 0 CP A,D
   BB       04 00 M1R 4 ... 0 ... 0 ... 0 ... 0 ... 0 ... 0 CP A,E
   BC       04 00 M1R 4 ... 0 ... 0 ... 0 ... 0 ... 0 ... 0 CP A,H
   BD       04 00 M1R 4 ... 0 ... 0 ... 0 ... 0 ... 0 ... 0 CP A,L
   BE       07 00 M1R 4 MRD 3 ... 0 ... 0 ... 0 ... 0 ... 0 CP A,(HL)
   BF       04 00 M1R 4 ... 0 ... 0 ... 0 ... 0 ... 0 ... 0 CP A,A
   C0       11 05 M1R 5 MRD 3 MRD 3 ... 0 ... 0 ... 0 ... 0 RET NZ
   C1       10 00 M1R 4 MRD 3 MRD 3 ... 0 ... 0 ... 0 ... 0 POP BC
   C2L2H2   10 10 M1R 4 MRD 3 MRD 3 ... 0 ... 0 ... 0 ... 0 JP NZ,U16
   C3L1H1   10 00 M1R 4 MRD 3 MRD 3 ... 0 ... 0 ... 0 ... 0 JP U16
   C4L2H2   17 10 M1R 4 MRD 3 MRD 4 MWR 3 MWR 3 ... 0 ... 0 CALL NZ,U16
   C5       11 00 M1R 5 MWR 3 MWR 3 ... 0 ... 0 ... 0 ... 0 PUSH BC
   C6U2     07 00 M1R 4 MRD 3 ... 0 ... 0 ... 0 ... 0 ... 0 ADD A,U8
   C7       11 00 M1R 5 MWR 3 MWR 3 ... 0 ... 0 ... 0 ... 0 RST 00H
   C8       11 05 M1R 5 MRD 3 MRD 3 ... 0 ... 0 ... 0 ... 0 RET Z
   C9       10 00 M1R 4 MRD 3 MRD 3 ... 0 ... 0 ... 0 ... 0 RET
   CAL2H2   10 10 M1R 4 MRD 3 MRD 3 ... 0 ... 0 ... 0 ... 0 JP Z,U16
   
   opc:    operation code [hex]
           L1,H1,U1,S1 means first operand direct number or address
           L2,H2,U2,S2 means second operand direct number or address
           L3,H3,U3,S3 means third operand direct number or address
           H,L ... U16 high and low byte
           U   ... U8 unsigned byte
           S   ... S8 signed byte
   
   T0      normal instruction duration [T] always 2 decimal digits
   T1      instruction duration if condition not met [T] always 2 decimal digits
   
   MC1++   Machine cycle first is type,second is duration [T] always 1 decimal digit
           ...     unused
           M1R     M1 cycle
           MRD     memory read
           MWR     memory write
           IOR     IO read
           IOW     IO write
           NON     no external operation (internal computation)
           INT     interrupt cycle
   
   mnem    instruction text (mnemonic)
   

   • opc is used for the address in an array of pointers
   • mnemonic is used to select the proper function pointer, and operands type
   • T0 and T1 are used for instructions timing (this is enough for rough emulations)
   • MC1++ are used for correct MC timings (to implement correct hardware emulation and contentions timing)

   As you can see all the times are in [T] tacts.

2. Tacts per frame

   On ZX computers the screen is outputted with 50 Hz frequency triggering interrupt each frame. The time between two interrupts in CPU clock tacts is referred as tacts per frame. This value is defined by model of ZX (type of ULA) and CPU clock frequency. It can be measured and used to determine type of ZX by reading IR register and approximation.

   This value is very important as all the stuff your code is doing must fit into it. The frame timing itself is a bit more complicated and can be used to achieve border effects or even multitech techniques.

   IIRC here are some actual values:

   69888T ZXS 48k
   70908T ZXS 128K
   80384T Didaktik M,Didaktik Kompakt
   

Answer 4

The much more commonly used term for the same thing outside Eastern Europe might be T-State, which is (without the hyphen) used by the original Zilog manuals - Another term is Instruction Cycle, which commonly notes the minimal instruction timing of a Z80, which consists of four T-states.

Takt is also used in German, but here we commonly use it as a synonym for "clock".

Answer 5

A "tact" is surely a foreign word for what in English would be called a "tick" (of a clock).

I know this is an old question now, but I was quite surprised to read down and see English translations and metaphors as a "clock cycle", a "time", a "measure", a "beat", a "musical meter", a "phase", even a "piston stroke"! Oddly there was not even any mention of a "pendulum swing", which is the most obvious mechanical manifestation.

Even @PhilipKendall who came closest, had to say "Clock ticks as seen by the processor", vaguely implying that there may be "clock ticks" other than those seen by the processor, or some other way to see "clock ticks" in this context - in other words suggesting that a "tact" may be different or more specific than "tick" - but not being clear about it.

There's only one "clock" that could mean anything here - it's that part of the machinery of a microprocessor, that provides the fundamental driving pulse of energy, and which triggers the transition of the processor between the states defined as part of its design (including intermediary states, in the case of multi-cycle instructions).