Seven-Segment Digits (BASIC)

Code status: maintained BASIC subroutine. The source listing is included below. Supply the caller and inputs described here before using it. ROM-loaded standalone checks compare all ten digits with an independent seven-segment pixel oracle.

Audit execution record · Scope and remaining checks

Overview

Draw large seven-segment style digits using PLOT and DRAW. Each digit is 40×56 pixels (5×7 character cells), cell-aligned with a 1-pixel outline around each segment. Digit shapes defined as 10 compact DATA strings — each character represents one segment (1=lit, 0=off). Reusable across any game needing a large numeric display: countdowns, scores, timers.

Calling and replacing a digit

Set integer d in 0–9, px in 0–216 and py in 55–175 before GO SUB 2000. Sinclair BASIC’s PLOT coordinates start at the lower-left of its 256 × 176 graphics area. For the advertised cell separation, use multiples of eight for both px and py—for example, 104 and 168. Arbitrary positions can share attribute cells with neighbouring artwork.

Set INK, PAPER, BRIGHT, FLASH, OVER and INVERSE deliberately; normal drawing uses OVER 0 and INVERSE 0. The routine changes i, s$, the graphics cursor and the shared DATA pointer. It uses DATA lines 9000–9009; another data reader must RESTORE its own data before continuing.

It adds the lit segments. Drawing 1 over 8 does not erase the other six segments. Start with a clear rectangle, clear the old owned rectangle before replacing a digit, or redraw the relevant background. The standalone example can use CLS; a game should avoid clearing unrelated artwork.

Why cell-aligned?

The 1-pixel inset on every segment isn’t decorative — it makes each segment fit inside its own character cell. Because the Spectrum’s attribute system stores one INK/PAPER pair per 8×8 cell, two segments touching at a cell boundary would share an attribute byte and clash colours. The cell-aligned design keeps every segment in its own cell, so the digit can be redrawn in any colour by setting INK before calling — the segments occupy separate attribute cells under the stated alignment. Artwork outside that rectangle still shares the screen and needs its own colour plan.

PLOT and DRAW honour the current INK colour: the segment pixels light up in whatever ink you set before GO SUB 2000. The cell’s PAPER colour stays whatever it was; segments effectively paint on top.

Code

  10 REM Seven-segment digit renderer
  20 REM Pattern: reusable across games
  30 REM
  40 REM Usage:
  50 REM   LET d=5: LET px=100: LET py=140
  60 REM   INK 6: GO SUB 2000
  70 REM
  80 REM Parameters:
  90 REM   d  = digit value (0-9)
 100 REM   px = x pixel coordinate (left edge)
 110 REM   py = y pixel coordinate (top edge)
 120 REM   INK colour set before calling
 130 REM
 140 REM Dimensions: 40x56 pixels (5x7 character cells)
 150 REM Each segment inset 1px from cell boundary
 160 REM Segment gap: 1 character cell (8 pixels)
 170 REM
 180 REM For two digits centred on screen:
 190 REM   Digit 1: px=92, Digit 2: px=140 (gap=8)
 200 REM For one digit centred: px=108
 210 REM
2000 REM === Draw digit d at px,py ===
2010 RESTORE 9000+d
2020 READ s$
2030 IF s$(1)="1" THEN FOR i=1 TO 6: PLOT px+9,py-i: DRAW 21,0: NEXT i
2040 IF s$(2)="1" THEN FOR i=1 TO 6: PLOT px+i,py-9: DRAW 0,-13: NEXT i
2050 IF s$(3)="1" THEN FOR i=1 TO 6: PLOT px+33+i,py-9: DRAW 0,-13: NEXT i
2060 IF s$(4)="1" THEN FOR i=1 TO 6: PLOT px+9,py-25-i: DRAW 21,0: NEXT i
2070 IF s$(5)="1" THEN FOR i=1 TO 6: PLOT px+i,py-33: DRAW 0,-13: NEXT i
2080 IF s$(6)="1" THEN FOR i=1 TO 6: PLOT px+33+i,py-33: DRAW 0,-13: NEXT i
2090 IF s$(7)="1" THEN FOR i=1 TO 6: PLOT px+9,py-49-i: DRAW 21,0: NEXT i
2100 RETURN
9000 DATA "1110111"
9001 DATA "0010010"
9002 DATA "1011101"
9003 DATA "1011011"
9004 DATA "0111010"
9005 DATA "1101011"
9006 DATA "1101111"
9007 DATA "1010010"
9008 DATA "1111111"
9009 DATA "1111011"

Trade-offs

When to use: Large digit displays — countdowns, scores, timers, PIN entry.

When to avoid: Small text — use PRINT for anything under 16px tall.

Related

Patterns: Numeric Display (assembly), Progress Bar

Vault: ULA — display memory and attribute clash | ZX Spectrum