A dialogue box needs line breaks that fit its width without requiring a second copy of the text.
Use it in a project
A byte buffer holds COUNT 0–16 ASCII characters. From START≤COUNT, skip leading spaces and choose up to WIDTH 1–16 characters. Prefer the last space before the hard limit; otherwise split a long word. Return LENGTH and NEXT; the visible slice starts at the possibly advanced START. NEXT skips following spaces. No font measurement or newline interpretation is performed.
Include component.inc once (MOS callers include the shared 6502 implementation through it). Reserve the public fields in this exact order: LENGTH, NEXT, VALID, START, WIDTH, COUNT, C0, C1, C2, C3, C4, C5, C6, C7, C8, C9, C10, C11, C12, C13, C14, C15. They occupy 22 contiguous bytes. Also reserve the private workspace: T0, T1, T2, T3 (4 bytes). Array names in the explanation refer to the numbered consecutive bindings, such as C0, C1 and so on.
Call bounded_text_wrap_init once at the intended reset boundary. It clears inputs as well as outputs and workspace. Set your starting state and configuration afterwards, then call bounded_text_wrap_step whenever the described event occurs. The helper reads no device, waits for no frame and installs no interrupt handler.
| Ownership | Fields |
|---|---|
| Retain between calls | buffer and COUNT |
| Supply before service | START and WIDTH |
| Read after service | updated START, LENGTH, NEXT and VALID |
Remember the most recent space while scanning to the width limit. If the next character continues a word, retreat to that space; otherwise keep the whole measured span.
Reusable initialisation and service
; SPDX-License-Identifier: MIT
; Choose a Bounded Text Line. See README.md for the byte interface and ownership.
; init clears the public fields and private workspace; set inputs afterwards.
bounded_text_wrap_init:
ld a,0
ld (LENGTH),a
ld (NEXT),a
ld (VALID),a
ld (START),a
ld (WIDTH),a
ld (COUNT),a
ld (C0),a
ld (C1),a
ld (C2),a
ld (C3),a
ld (C4),a
ld (C5),a
ld (C6),a
ld (C7),a
ld (C8),a
ld (C9),a
ld (C10),a
ld (C11),a
ld (C12),a
ld (C13),a
ld (C14),a
ld (C15),a
ld (T0),a
ld (T1),a
ld (T2),a
ld (T3),a
ret
bounded_text_wrap_step:
; LENGTH = 0
ld a,0
ld (LENGTH),a
; NEXT = START
ld a,(START)
ld (NEXT),a
; VALID = 0
ld a,0
ld (VALID),a
; if COUNT > 16:
ld a,(COUNT)
cp 16
jp z,bounded_text_wrap_L2
jp nc,bounded_text_wrap_L1
jp bounded_text_wrap_L2
bounded_text_wrap_L1:
; return
ret
jp bounded_text_wrap_L3
bounded_text_wrap_L2:
bounded_text_wrap_L3:
; if START > COUNT:
ld a,(START)
ld b,a
ld a,(COUNT)
ld c,a
ld a,b
cp c
jp z,bounded_text_wrap_L5
jp nc,bounded_text_wrap_L4
jp bounded_text_wrap_L5
bounded_text_wrap_L4:
; return
ret
jp bounded_text_wrap_L6
bounded_text_wrap_L5:
bounded_text_wrap_L6:
; if WIDTH == 0:
ld a,(WIDTH)
cp 0
jp z,bounded_text_wrap_L7
jp bounded_text_wrap_L8
bounded_text_wrap_L7:
; return
ret
jp bounded_text_wrap_L9
bounded_text_wrap_L8:
bounded_text_wrap_L9:
; if WIDTH > 16:
ld a,(WIDTH)
cp 16
jp z,bounded_text_wrap_L11
jp nc,bounded_text_wrap_L10
jp bounded_text_wrap_L11
bounded_text_wrap_L10:
; return
ret
jp bounded_text_wrap_L12
bounded_text_wrap_L11:
bounded_text_wrap_L12:
; VALID = 1
ld a,1
ld (VALID),a
; while START < COUNT:
bounded_text_wrap_L13:
ld a,(START)
ld b,a
ld a,(COUNT)
ld c,a
ld a,b
cp c
jp c,bounded_text_wrap_L14
jp bounded_text_wrap_L15
bounded_text_wrap_L14:
; T0 = C[START]
ld a,(START)
ld e,a
ld d,0
ld hl,C0
add hl,de
ld a,(hl)
ld (T0),a
; if T0 != 32:
ld a,(T0)
cp 32
jp nz,bounded_text_wrap_L16
jp bounded_text_wrap_L17
bounded_text_wrap_L16:
; break
jp bounded_text_wrap_L15
jp bounded_text_wrap_L18
bounded_text_wrap_L17:
bounded_text_wrap_L18:
; START = START + 1
ld a,(START)
add a,1
ld (START),a
jp bounded_text_wrap_L13
bounded_text_wrap_L15:
; NEXT = START
ld a,(START)
ld (NEXT),a
; T0 = START
ld a,(START)
ld (T0),a
; T1 = 255
ld a,255
ld (T1),a
; while T0 < COUNT:
bounded_text_wrap_L19:
ld a,(T0)
ld b,a
ld a,(COUNT)
ld c,a
ld a,b
cp c
jp c,bounded_text_wrap_L20
jp bounded_text_wrap_L21
bounded_text_wrap_L20:
; T2 = T0 - START
ld a,(T0)
ld b,a
ld a,(START)
ld c,a
ld a,b
sub c
ld (T2),a
; if T2 == WIDTH:
ld a,(T2)
ld b,a
ld a,(WIDTH)
ld c,a
ld a,b
cp c
jp z,bounded_text_wrap_L22
jp bounded_text_wrap_L23
bounded_text_wrap_L22:
; break
jp bounded_text_wrap_L21
jp bounded_text_wrap_L24
bounded_text_wrap_L23:
bounded_text_wrap_L24:
; T2 = C[T0]
ld a,(T0)
ld e,a
ld d,0
ld hl,C0
add hl,de
ld a,(hl)
ld (T2),a
; if T2 == 32:
ld a,(T2)
cp 32
jp z,bounded_text_wrap_L25
jp bounded_text_wrap_L26
bounded_text_wrap_L25:
; T1 = T0
ld a,(T0)
ld (T1),a
jp bounded_text_wrap_L27
bounded_text_wrap_L26:
bounded_text_wrap_L27:
; T0 = T0 + 1
ld a,(T0)
add a,1
ld (T0),a
jp bounded_text_wrap_L19
bounded_text_wrap_L21:
; T2 = T0
ld a,(T0)
ld (T2),a
; if T0 < COUNT:
ld a,(T0)
ld b,a
ld a,(COUNT)
ld c,a
ld a,b
cp c
jp c,bounded_text_wrap_L28
jp bounded_text_wrap_L29
bounded_text_wrap_L28:
; T3 = C[T0]
ld a,(T0)
ld e,a
ld d,0
ld hl,C0
add hl,de
ld a,(hl)
ld (T3),a
; if T3 != 32:
ld a,(T3)
cp 32
jp nz,bounded_text_wrap_L31
jp bounded_text_wrap_L32
bounded_text_wrap_L31:
; if T1 != 255:
ld a,(T1)
cp 255
jp nz,bounded_text_wrap_L34
jp bounded_text_wrap_L35
bounded_text_wrap_L34:
; T2 = T1
ld a,(T1)
ld (T2),a
jp bounded_text_wrap_L36
bounded_text_wrap_L35:
bounded_text_wrap_L36:
jp bounded_text_wrap_L33
bounded_text_wrap_L32:
bounded_text_wrap_L33:
jp bounded_text_wrap_L30
bounded_text_wrap_L29:
bounded_text_wrap_L30:
; LENGTH = T2 - START
ld a,(T2)
ld b,a
ld a,(START)
ld c,a
ld a,b
sub c
ld (LENGTH),a
; NEXT = T2
ld a,(T2)
ld (NEXT),a
; while NEXT < COUNT:
bounded_text_wrap_L37:
ld a,(NEXT)
ld b,a
ld a,(COUNT)
ld c,a
ld a,b
cp c
jp c,bounded_text_wrap_L38
jp bounded_text_wrap_L39
bounded_text_wrap_L38:
; T3 = C[NEXT]
ld a,(NEXT)
ld e,a
ld d,0
ld hl,C0
add hl,de
ld a,(hl)
ld (T3),a
; if T3 != 32:
ld a,(T3)
cp 32
jp nz,bounded_text_wrap_L40
jp bounded_text_wrap_L41
bounded_text_wrap_L40:
; break
jp bounded_text_wrap_L39
jp bounded_text_wrap_L42
bounded_text_wrap_L41:
bounded_text_wrap_L42:
; NEXT = NEXT + 1
ld a,(NEXT)
add a,1
ld (NEXT),a
jp bounded_text_wrap_L37
bounded_text_wrap_L39:
ret
component_end:Machine contract
AF/BC/DE/HL may change; IX/IY are preserved. A direct CALL needs two return-address stack bytes. The helper makes no nested calls and adds no other stack use. Its named scratch is private workspace, not a pointer, a ROM variable or shared zero-page space that the game may borrow concurrently. The samples use fixed symbol bindings. Scope or rename every label and binding, including the component_end measurement marker, and allocate a separate block for each independent instance. Different helpers can also use common field names, so combining them needs separate namespaces or explicitly renamed bindings. Give each block one mutation owner.
The 48K diagnostic boots through ROM tape loading and USR, disables interrupts and owns the screen. The helper performs no port or screen writes.
Adapt the caller
demo.asm supplies the bindings and include order. exercise.inc first runs table cases, then a separate client retaining state between calls. A game can call the public step entry directly and remove the diagnostic packet and display. Do not copy the diagnostic’s hardware takeover into an existing game loop.
Build with make ASM198X=/path/to/asm198x. The Makefile produces normal, custom, deliberately wrong and repaired programs. Tools must already be installed; the sample downloads nothing.
Bindings and include order
org $8000
CUSTOM equ 0
BROKEN equ 0
COYOTE_STEPS equ 4
BUFFER_STEPS equ 4
TEXT_CAP equ 8
PROPERTY_COUNT equ 16
STREAM_BYTES equ 13
CRC_POLY equ 7
include "caller.inc"
include "exercise.inc"
component_start:
include "component.inc"
include "data.inc"
include "cases.inc"
include "../../physics/rectangle-overlap/display.inc"
code_end:
STATE:
LENGTH: defb 0
NEXT: defb 0
VALID: defb 0
START: defb 0
WIDTH: defb 0
COUNT: defb 0
C0: defb 0
C1: defb 0
C2: defb 0
C3: defb 0
C4: defb 0
C5: defb 0
C6: defb 0
C7: defb 0
C8: defb 0
C9: defb 0
C10: defb 0
C11: defb 0
C12: defb 0
C13: defb 0
C14: defb 0
C15: defb 0
T0: defb 0
T1: defb 0
T2: defb 0
T3: defb 0
input_ptr: defw 0
output_ptr: defw 0
cases_left: defb 0
ready: defb 0
preview: defs 8,0
OUTPUT: defs 1300,0
end startA client retaining state between calls
preview_rules:
if CUSTOM
; COUNT = 8
ld a,8
ld (COUNT),a
; WIDTH = 3
ld a,3
ld (WIDTH),a
; C0 = 65
ld a,65
ld (C0),a
; C1 = 66
ld a,66
ld (C1),a
; C2 = 67
ld a,67
ld (C2),a
; C3 = 68
ld a,68
ld (C3),a
; C4 = 69
ld a,69
ld (C4),a
; C5 = 70
ld a,70
ld (C5),a
; C6 = 71
ld a,71
ld (C6),a
; C7 = 72
ld a,72
ld (C7),a
if BROKEN
; WIDTH = 1
ld a,1
ld (WIDTH),a
endif
call checked_step
call emit_state
ret
else
; COUNT = 11
ld a,11
ld (COUNT),a
; WIDTH = 7
ld a,7
ld (WIDTH),a
; C0 = 72
ld a,72
ld (C0),a
; C1 = 69
ld a,69
ld (C1),a
; C2 = 76
ld a,76
ld (C2),a
; C3 = 76
ld a,76
ld (C3),a
; C4 = 79
ld a,79
ld (C4),a
; C5 = 32
ld a,32
ld (C5),a
; C6 = 87
ld a,87
ld (C6),a
; C7 = 79
ld a,79
ld (C7),a
; C8 = 82
ld a,82
ld (C8),a
; C9 = 76
ld a,76
ld (C9),a
; C10 = 68
ld a,68
ld (C10),a
if BROKEN
; WIDTH = 1
ld a,1
ld (WIDTH),a
endif
call checked_step
call emit_state
endif
retCustomise it
Hard-wrap a word longer than the three-character line width. The maintained custom caller changes its inputs or client sequence rather than hiding a different implementation behind the same label. Compare every state of the two clients, not only their final display.
Custom client selection
org $8000
CUSTOM equ 1
BROKEN equ 0
COYOTE_STEPS equ 2
BUFFER_STEPS equ 2
TEXT_CAP equ 4
PROPERTY_COUNT equ 16
STREAM_BYTES equ 13
CRC_POLY equ 49
include "caller.inc"
include "exercise.inc"
component_start:
include "component.inc"
include "data.inc"
include "cases.inc"
include "../../physics/rectangle-overlap/display.inc"
code_end:
STATE:
LENGTH: defb 0
NEXT: defb 0
VALID: defb 0
START: defb 0
WIDTH: defb 0
COUNT: defb 0
C0: defb 0
C1: defb 0
C2: defb 0
C3: defb 0
C4: defb 0
C5: defb 0
C6: defb 0
C7: defb 0
C8: defb 0
C9: defb 0
C10: defb 0
C11: defb 0
C12: defb 0
C13: defb 0
C14: defb 0
C15: defb 0
T0: defb 0
T1: defb 0
T2: defb 0
T3: defb 0
input_ptr: defw 0
output_ptr: defw 0
cases_left: defb 0
ready: defb 0
preview: defs 8,0
OUTPUT: defs 1300,0
end startThe custom diagnostic row is 03 03 01 00 03 08 41 42. The full client trace in the source explains how it gets there.
Mistake and repair
The wrong caller supplies a one-column width. Ordinary words break into individual letters. Pass the text area’s character width, not a pixel count or an unrelated default.
The BROKEN branch lives in the sequential client, not the reusable helper. wrong.asm selects it; repaired.asm restores the normal call sequence. The repaired executable matches the normal program (for Spectrum, compare loaded CODE rather than the tape filename).
Select the deliberately wrong client
org $8000
CUSTOM equ 0
BROKEN equ 1
COYOTE_STEPS equ 4
BUFFER_STEPS equ 4
TEXT_CAP equ 8
PROPERTY_COUNT equ 16
STREAM_BYTES equ 13
CRC_POLY equ 7
include "caller.inc"
include "exercise.inc"
component_start:
include "component.inc"
include "data.inc"
include "cases.inc"
include "../../physics/rectangle-overlap/display.inc"
code_end:
STATE:
LENGTH: defb 0
NEXT: defb 0
VALID: defb 0
START: defb 0
WIDTH: defb 0
COUNT: defb 0
C0: defb 0
C1: defb 0
C2: defb 0
C3: defb 0
C4: defb 0
C5: defb 0
C6: defb 0
C7: defb 0
C8: defb 0
C9: defb 0
C10: defb 0
C11: defb 0
C12: defb 0
C13: defb 0
C14: defb 0
C15: defb 0
T0: defb 0
T1: defb 0
T2: defb 0
T3: defb 0
input_ptr: defw 0
output_ptr: defw 0
cases_left: defb 0
ready: defb 0
preview: defs 8,0
OUTPUT: defs 1300,0
end start
Cost and alternatives
The measured helper extent is 498 code bytes; public state is 22 bytes and private scratch is 4 bytes. There are no immutable helper tables. Caller data, the test cases, return-address stack, alignment and display buffers cost additional space. These are assembled sizes, not cycle measurements.
At most sixteen characters plus skipped spaces, without copying. A proportional font needs a pixel-width accumulator instead of one count per character.
This handles fixed-width ASCII spaces. Proportional fonts, explicit newlines, Unicode and hyphenation need richer rules. The caller must set START=NEXT before requesting the next line.
Execution evidence
Four variants run on 48K PAL ROM tape load and USR, using Asm198x 0.0.72, Emu198x 0.44.2. Each executes 45 boundary/varied/error table cases, dirty initialisation and the separate retained-state client. An independent mathematical/container model checks every public byte after every call. This is bounded example coverage, not exhaustive correctness for every possible state.
The records retain loaded immutable code, completion, register sentinels, full packets, source/tool/firmware hashes and captures. The public gate validates these retained observations and rejects altered evidence; it does not run restricted firmware. No firmware is distributed. The eight displayed bytes are the first eight public fields, zero-padded when shorter; the packet includes every field.
Existing alternatives
The existing text layout and word-aware panel includes layout and drawing on its documented machines. Reuse that complete panel when it fits your renderer. This short-span selector is useful when a game already owns its drawing code and only needs the next source span: its sixteen-byte buffer, character-count width and returned indices keep that boundary explicit.
Where it fits
Text entry buffers: Keep editing and layout separate so reflow does not mutate the original text.