Sinclair BASIC hardware
The screen, colour, graphics and sound hardware of each machine that runs Sinclair BASIC, and where machine-code and data blocks live in its memory. A ZX81 and a Spectrum 128 share a BASIC and almost no hardware at all, so each machine has a section of its own.
Sinclair ZX81
Screen modes
The ZX81 has a single 32×24 character display, with the bottom two lines reserved for input and reports. What it does have is two speed modes: SLOW keeps the picture on screen continuously with the CPU running at about a quarter speed, while FAST blanks the screen for full-speed computation, flickering it on only during INPUT or PAUSE. When the screen is full, SCROLL moves the whole display up a line — printing past the bottom without it stops the program with report 5.
Colour
The ZX81 has no colour hardware — the display is black on white. Individual characters can be shown in inverse video (white on black) using the inverse character set (see the escape codes page).
Graphics
There is no bitmap mode. PLOT and UNPLOT set and clear block pixels on a low-resolution grid — x 0–63, y 0–43, origin at the bottom-left — where each character cell is a 2×2 group of block pixels drawn with the charset's block-graphics characters. The same characters can be printed directly for chunky graphics (see escape codes).
Sound
The ZX81 has no sound hardware.
Memory
The whole of the machine's address space, region by region. Zoom in to open a band into the parts it groups, and select a region for its addresses and what sits there.
A ZX81 program can carry machine code or data — memory blocks — using the classic trick of hiding the bytes inside a REM line. Because a .P file holds only the BASIC program, this is the one place code can live that still travels in the single standard .P file that real emulators and hardware load: the bytes sit in the program itself.
Each hidden-code REM line shows in the editor as a block tab alongside the BASIC tab. Open the tab to edit the block's assembly; saving rewrites the hidden REM line for you, so the machine code always stays part of the program listing. Add a new block with the + button on the tab strip, or import a .P that already contains one — the IDE recognises the hidden code and gives it a tab.
A block's address is fixed by where its REM line sits in the program. The first line's REM body lands at the famous 16514, so a block placed there is reached with RAND USR 16514 (or PRINT PEEK 16514). The address is shown in the block tab and can't be typed in — move the REM line to change it. You can mark a block as code or data and give it a name; those labels are remembered in the project bundle.
Because the bytes live in the listing, they export and import with the ordinary .P file — no separate file, and it runs on a real ZX81 unchanged.
See the machine code guide and the cross-dialect Machine code & data blocks overview. Every mnemonic, directive and operand form the assembly editor accepts is in the Z80 assembly reference.
ZX Spectrum 48K
Screen modes
The Spectrum has a single display mode: a 256×192 pixel bitmap overlaid by a 32×24 grid of colour attribute cells. The top 22 character rows hold program output; the bottom two lines are reserved for input and reports.
Colour
Eight colours (0 black to 7 white) are available for INK (foreground), PAPER (background) and BORDER, with two per-cell modifiers: BRIGHT for high intensity and FLASH to alternate ink and paper. Colour is stored one ink/paper pair per 8×8 attribute cell, so two differently-coloured shapes in the same cell clash — the classic Spectrum attribute clash. ATTR reads a cell's attribute byte back; INVERSE and OVER modify how following output is drawn.
The screen here is the 256 × 192 active display and nothing around it, so BORDER sets a colour that is not drawn. It still stores and reads back as it does on the machine, and a program that uses the border for a loading stripe or a timing flash runs — the flash is simply not visible.
Graphics
PLOT <x>, <y> sets a pixel — x runs 0–255 and y 0–175 from the bottom-left. DRAW <dx>, <dy> draws a line by a relative offset — unlike most machines here, which draw to an absolute point — and a third argument bends it into an arc. CIRCLE <x>, <y> takes an absolute centre and a radius, and the POINT function tests whether a pixel is set.
Sound
BEEP <duration>, <pitch> plays a tone through the internal beeper — the duration in seconds and the pitch in semitones above or below middle C.
Timing
The Spectrum's processor runs at 3.5 MHz over 312 screen lines a frame, just over fifty frames a second.
Not all of that time is the program's. The display chip reads the picture out of the same 16K of memory the processor uses — everything from 16384 to 32767, which is where the screen, the BASIC program and its variables all live — and while it is fetching a line it holds the processor off the memory for up to six clock cycles at a time. Over a frame that is a real slice of the machine: an identical routine runs several per cent faster from 32768 upwards, where the display chip never looks, than it does from below. It is why a delay written as FOR n=1 TO 1000 is worth timing rather than calculating, and why moving a machine-code routine above 32768 speeds it up for nothing.
The same sharing is what makes the Spectrum's multicolour effects possible. Because the processor is held off in a pattern that repeats with the picture, a routine that rewrites colour attributes as the screen is drawn is pulled into step with the beam on every pass, and its coloured bands hold still instead of sliding. A routine timed on a real Spectrum keeps that relationship here.
Fifty times a second the display hardware interrupts the processor, and it holds the request there for a moment rather than raising it for a single instant — so a routine that has interrupts switched off just as a frame turns over still receives that frame's interrupt once it switches them back on, provided it does so promptly. Leave them off for longer and the frame's interrupt is missed, as it is on the machine.
Two things this does not reproduce. The floating bus is not modelled, so the trick of reading port 0xFF to find the beam answers nothing useful — a routine that syncs that way will not. And contention is charged per memory access rather than per processor cycle, which is a few clock cycles out inside a single instruction; the error cannot accumulate, and it is far below anything the picture can show.
Memory
The whole of the machine's address space, region by region. Zoom in to open a band into the parts it groups, and select a region for its addresses and what sits there.
A ZX Spectrum program can carry fixed-address machine code or data — memory blocks — that load into RAM alongside the BASIC program and are in place before it runs. A block may sit anywhere in the RAM above the ROM, from 0x4000 to 0xFFFF; new blocks default to 0x8000, clear of a typical program and its variables. This holds for both the 48K and 128K models.
Two regions are flagged with a warning rather than refused: the display file and colour attributes at 0x4000–0x5AFF, and the system-variable area just above it. A block there loads, but the running machine may overwrite it.
When a block sits below the default RAMTOP, the IDE runs a CLEAR for the byte just below the block before it starts the program, so the BASIC stack can't grow up over your code — poke a routine at 32768 and the IDE issues CLEAR 32767 first.
Blocks travel with the document through the project bundle and through share links. They can also arrive on import: a .TAP containing CODE files brings each CODE file in as a block. A tape that uses a tiny loader to chain into a larger program is recognised — the loader is skipped (with a note) and the real program imported. On Run the IDE refuses to start if a block would overlap the BASIC program, and warns (but allows) a block over reserved hardware.
For a worked example, poke the five bytes 3E 02 D3 FE C9 (LD A,2 : OUT (0xFE),A : RET) at 32768 and add 10 RANDOMIZE USR 32768: running it turns the border red. See the machine code guide for the full how-to and Machine code & data blocks for the cross-dialect overview. Every mnemonic, directive and operand form the assembly editor accepts is in the Z80 assembly reference.
ZX Spectrum 128K
Screen modes
The display hardware is the 48K's: the same single 256×192 bitmap mode with 32×24 attribute cells.
Colour
Identical to the 48K — the same eight colours, BRIGHT, FLASH and per-cell attributes.
Graphics
Identical to the 48K — the same PLOT/DRAW/CIRCLE commands and coordinate space.
Sound
Alongside the 48K beeper and BEEP, the 128K models add an AY-3-8912 three-channel sound chip, driven from BASIC with PLAY — one music string per channel. Keywords tagged 128K only in the reference table, such as PLAY, need 128 BASIC mode.
Timing
The 128K shares the picture out the same way the 48K does, on a slightly different clock: 3.5469 MHz over 311 lines a frame. The memory from 16384 to 32767 is held off while the display is drawn, exactly as above — and so is whichever of the extra memory banks a program has switched into the top 16K, if it is an odd-numbered one. The same routine can therefore run at two different speeds at 49152 depending only on which bank is switched in there.
Memory
The whole of the machine's address space, region by region. Zoom in to open a band into the parts it groups, and select a region for its addresses and what sits there.
Block placement is identical to the 48K: the same 0x4000–0xFFFF window, default address, warnings and CLEAR handling described above apply to the 128K models.