Code Like It's 198x

Text Utilities (BASIC)

Centre text on any row and clear rows — two tiny subroutines used in every BASIC game.

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 now cover floor centring, empty/full-width text and the clear-row footprint.

Audit execution record · Scope and remaining checks

Overview

Two subroutines that appear in every Spectrum BASIC game. Centre text calculates the column from the string length and prints at the midpoint. Clear row overwrites a row with spaces. Together they handle all status messages, titles, and feedback text without manual column counting.

Calling the routines

For GO SUB 3000, supply integer row r in 0–21 and a printable string t$ of length 0–32. Integer division leaves an odd spare column on the right. The routine does not clear an earlier, longer message: clear the owned area before replacing it.

For GO SUB 3100, supply r in 0–21. It clears exactly columns 2–29, leaving the surrounding columns alone. It inherits INK, PAPER, BRIGHT and FLASH; choose them before calling. Neither routine reserves a temporary variable, but both change the print cursor and display.

Code

text-utilities.bas
  10 REM Text utilities
  20 REM Pattern: reusable across games
  30 REM
  40 REM Centre text:
  50 REM   LET r=10: LET t$="Hello": GO SUB 3000
  60 REM   Set INK, BRIGHT, FLASH etc before calling
  70 REM
  80 REM Clear row (cols 2-29, inside a frame):
  90 REM   LET r=10: GO SUB 3100
 100 REM
3000 REM === Centre text t$ on row r ===
3010 PRINT AT r,INT ((32-LEN t$)/2);t$
3020 RETURN
3100 REM === Clear row r (cols 2-29) ===
3110 PRINT AT r,2;"                            "
3120 RETURN

Trade-offs

Aspect Detail
Speed Two PRINT AT calls — negligible overhead
Memory 4 lines of code total
Limitation Clear row writes cols 2–29 (inside a frame). Adjust for full-width
Flexibility Set INK, BRIGHT, FLASH before calling — the subroutine inherits them

When to use: Every game. Title screens, feedback messages, score displays, prompts.

When another approach helps: Dense HUD values often need stable left or right edges rather than centring. Clear only the area owned by that message.

Patterns: Screen Frame, Character Printing (assembly)

Vault: ULA | ZX Spectrum