The ULA

<colophon-ula> shows the Ferranti uncommitted logic array, which on a Spectrum is the whole board but the processor and the memory. It counts out the frame, reads the screen, turns bytes into pixels, raises the interrupt, holds the border colour and drives the speaker — so where a CPC’s video wants two panels, this machine wants one.

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It bears the heading ULA Ferranti 5C102E and redraws whenever the machine says something changed.

What it shows

Frame T-state is the chip’s own clock: T-states since the interrupt, which is the unit every published timing for this machine is given in, and the reason a Spectrum program’s speed is a property of where in the frame it began. Border is the three bits of the last write to port &FE.

Both are editable, so a frame can be stood at a chosen T-state and a border set from the panel. Writing the frame T-state moves the beam with it: the chip’s line and column are worked out from it rather than kept beside it, so they cannot be set to disagree.

Line and Column are where the beam stands, Frame is frames since reset — FLASH swaps ink and paper on its bit 4 — and EAR and MIC are the two bits of that same port write that reach the sockets. EAR is what the speaker follows; MIC is what the machine writes to tape, and it goes nowhere here.

What it cannot show

The chip fetches ahead of the beam through a pipeline the emulator collapses into a single T-state, and that pipeline is what a floating-bus read observes. So the value an unattached port returns is not among the things this panel can be asked for, because it is not among the things the chip is modelled as holding.