The Gate Array

<colophon-gate-array> shows what the Gate Array is holding. The chip takes commands and answers none of them: a program that wants to know its own palette must remember having written it. So unlike every other panel here, this one shows values nothing on the machine can ask for.

<colophon-gate-array></colophon-gate-array>

The inks

Sixteen pens and the border, each a hardware colour code with the colour it makes beside it. These are the same codes a screen is given to read a region by, so a palette worth declaring on the page can be read off the machine here first.

A code typed into one lands where an INKR write would have left it, and the picture is repainted around it. Five bits is all a code carries, and the field refuses anything wider.

The colour beside a code is the other way to set it. Pressing it opens the thirty-two the chip can make, each under the code that names it and the standing one marked, and choosing one writes that code exactly as typing it would. A few of the thirty-two stand almost on top of one another: the palette is measured off a real 40010 rather than derived from its logic, and they are distinct codes all the same.

The mode

MODE is the mode the picture is being drawn in. ASKED is the mode as last written, which comes into force after the next line sync — the delay that a mode split is built on. While the two differ, a change is in flight and has not yet been shown.

The ROMs

LOWER and UPPER say whether the firmware stands over &0000 to &3FFF and over &C000 to &FFFF. They are the answer to the difference the memory panel reports between what the processor sees and what the banks hold.

Turning one off here pages that ROM out as a program’s own write would, and what the processor reads changes under it while the bytes in the bank stay where they were.

The interrupts

R52 counts the line syncs since the last interrupt; the fifty-second raises one. INT is a request made and not yet taken — it is held until the processor acknowledges it, so a machine running with interrupts disabled shows one standing. Whether the processor is listening at all is the Z80’s to say.