// HARDWARE · WHAT'S UNDERNEATH //

The register is the bitfield

chmod 755. A pile of feature flags packed into one integer. flags |= ENABLED, flags &= ~VISIBLE. You pack booleans into bits and never quite ask why it feels so natural. On the ZX Spectrum the hardware gives you no choice — a single byte to one port runs the border, the speaker and the tape at once.

One port, several switches

The Spectrum’s ULA — the chip that does very nearly everything — listens on I/O port $FE. The byte you write there isn’t a number; it’s a row of independent switches, addressed by position:

Z80 · ONE BYTE, THREE JOBS

; bit:  7 6 5   4         3        2 1 0
;             speaker    tape     border colour
      ld   a, %00010010   ; speaker on, border = 010 (red)
      out  ($fe), a

Three unrelated pieces of hardware, selected by which bits you set in one byte.

Which is why you mask

To flip one switch without disturbing the rest, you don’t write a fresh number — you OR a bit in, or AND it out, against the byte you already have:

Z80 · SET AND CLEAR ONE BIT

        ld   a, (shadow)    ; the byte we last sent
      or   %00010000      ; speaker bit ON  — leave the rest alone
      out  ($fe), a
      and  %11101111      ; speaker bit OFF — leave the rest alone
      out  ($fe), a

OR to set, AND with the inverse to clear. Set, clear, test — the three bitmask moves, in hardware.

That is flags |= SPEAKER and flags &= ~SPEAKER, forty years early. chmod 755 is the same idea — three rwx bits per user, packed and masked — and so is every options-bitfield, permission set and packed-flags enum you’ve ever touched.

Booleans-in-bits isn’t a trick programmers invented to look clever. It’s the shape the hardware shipped in. The Spectrum’s ULA port makes the lesson literal: a bitmask is simply how you address a row of switches that happen to share one byte.

>Every great programmer started with one instruction.