Centre Text on a Character Screen

Overview

Centred text is a calculation, not a collection of hand-tuned coordinates:

left column = (screen width - text length) / 2

Integer division naturally puts an odd spare column on the right. That is usually the least surprising convention, and using one convention everywhere matters more than moving individual messages until they look approximately right.

The routine should also define what happens when text is as wide as, or wider than, the screen. Starting at column zero is a safe fallback for fixed-width displays: it avoids unsigned subtraction wrapping into a position far beyond the visible row.

6502 implementation

This ca65-compatible version targets a null-terminated string and a character screen whose existing print_string routine reads text_ptr and advances from cursor_x, cursor_y.

SCREEN_WIDTH = 22

; Input: text_ptr = address of null-terminated string
;        A        = destination row
; Output: string printed; cursor left immediately after it
; Clobbers: A, Y, text_length
print_centered:
        sta cursor_y
        ldy #0

count:
        lda (text_ptr),y
        beq have_length
        iny
        cpy #SCREEN_WIDTH
        bcs full_width
        bne count

have_length:
        sty text_length
        lda #SCREEN_WIDTH
        sec
        sbc text_length
        lsr                     ; divide unused columns by two
        sta cursor_x
        jmp print_string

full_width:
        lda #0
        sta cursor_x
        jmp print_string

Example:

        lda #<title
        sta text_ptr
        lda #>title
        sta text_ptr+1
        lda #4                  ; row
        jsr print_centered

title:
        .byte "RACHEL", 0

Why measure at runtime?

Hard-coding (width - 6) / 2 for RACHEL saves a small loop, but silently breaks when a writer changes the text. Runtime measurement keeps content and layout independent, which is particularly useful for status, lobby and result screens that evolve late in a game’s development.

On memory-constrained hardware, store a known length beside each string and pass it to the centring routine instead. The positioning formula stays the same; only the measurement strategy changes.

Trade-offs

Aspect	Cost
CPU	One linear scan before printing
Memory	Roughly 30 bytes on 6502, plus one temporary byte
Limitation	Centres characters, not proportional pixel widths
Failure policy	Full-width and oversized strings begin at column zero

Use it for: titles, prompts, lobby states, errors and result screens.

Avoid it when: multiple values must align by a shared column, or the text is part of a dense HUD where stable edges are easier to scan than centred labels.