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:
lda #0
sta LENGTH
sta NEXT
sta VALID
sta START
sta WIDTH
sta COUNT
sta C0
sta C1
sta C2
sta C3
sta C4
sta C5
sta C6
sta C7
sta C8
sta C9
sta C10
sta C11
sta C12
sta C13
sta C14
sta C15
sta T0
sta T1
sta T2
sta T3
rts
bounded_text_wrap_step:
; LENGTH = 0
lda #0
sta LENGTH
; NEXT = START
lda START
sta NEXT
; VALID = 0
lda #0
sta VALID
; if COUNT > 16:
lda COUNT
cmp #16
beq bounded_text_wrap_L5
bcs bounded_text_wrap_L4
bounded_text_wrap_L5:
jmp bounded_text_wrap_L2
bounded_text_wrap_L4:
jmp bounded_text_wrap_L1
bounded_text_wrap_L1:
; return
rts
jmp bounded_text_wrap_L3
bounded_text_wrap_L2:
bounded_text_wrap_L3:
; if START > COUNT:
lda START
cmp COUNT
beq bounded_text_wrap_L10
bcs bounded_text_wrap_L9
bounded_text_wrap_L10:
jmp bounded_text_wrap_L7
bounded_text_wrap_L9:
jmp bounded_text_wrap_L6
bounded_text_wrap_L6:
; return
rts
jmp bounded_text_wrap_L8
bounded_text_wrap_L7:
bounded_text_wrap_L8:
; if WIDTH == 0:
lda WIDTH
cmp #0
beq bounded_text_wrap_L14
bounded_text_wrap_L15:
jmp bounded_text_wrap_L12
bounded_text_wrap_L14:
jmp bounded_text_wrap_L11
bounded_text_wrap_L11:
; return
rts
jmp bounded_text_wrap_L13
bounded_text_wrap_L12:
bounded_text_wrap_L13:
; if WIDTH > 16:
lda WIDTH
cmp #16
beq bounded_text_wrap_L20
bcs bounded_text_wrap_L19
bounded_text_wrap_L20:
jmp bounded_text_wrap_L17
bounded_text_wrap_L19:
jmp bounded_text_wrap_L16
bounded_text_wrap_L16:
; return
rts
jmp bounded_text_wrap_L18
bounded_text_wrap_L17:
bounded_text_wrap_L18:
; VALID = 1
lda #1
sta VALID
; while START < COUNT:
bounded_text_wrap_L21:
lda START
cmp COUNT
bcc bounded_text_wrap_L24
bounded_text_wrap_L25:
jmp bounded_text_wrap_L23
bounded_text_wrap_L24:
jmp bounded_text_wrap_L22
bounded_text_wrap_L22:
; T0 = C[START]
ldx START
lda C0,x
sta T0
; if T0 != 32:
lda T0
cmp #32
bne bounded_text_wrap_L29
bounded_text_wrap_L30:
jmp bounded_text_wrap_L27
bounded_text_wrap_L29:
jmp bounded_text_wrap_L26
bounded_text_wrap_L26:
; break
jmp bounded_text_wrap_L23
jmp bounded_text_wrap_L28
bounded_text_wrap_L27:
bounded_text_wrap_L28:
; START = START + 1
lda START
clc
adc #1
sta START
jmp bounded_text_wrap_L21
bounded_text_wrap_L23:
; NEXT = START
lda START
sta NEXT
; T0 = START
lda START
sta T0
; T1 = 255
lda #255
sta T1
; while T0 < COUNT:
bounded_text_wrap_L31:
lda T0
cmp COUNT
bcc bounded_text_wrap_L34
bounded_text_wrap_L35:
jmp bounded_text_wrap_L33
bounded_text_wrap_L34:
jmp bounded_text_wrap_L32
bounded_text_wrap_L32:
; T2 = T0 - START
lda T0
sec
sbc START
sta T2
; if T2 == WIDTH:
lda T2
cmp WIDTH
beq bounded_text_wrap_L39
bounded_text_wrap_L40:
jmp bounded_text_wrap_L37
bounded_text_wrap_L39:
jmp bounded_text_wrap_L36
bounded_text_wrap_L36:
; break
jmp bounded_text_wrap_L33
jmp bounded_text_wrap_L38
bounded_text_wrap_L37:
bounded_text_wrap_L38:
; T2 = C[T0]
ldx T0
lda C0,x
sta T2
; if T2 == 32:
lda T2
cmp #32
beq bounded_text_wrap_L44
bounded_text_wrap_L45:
jmp bounded_text_wrap_L42
bounded_text_wrap_L44:
jmp bounded_text_wrap_L41
bounded_text_wrap_L41:
; T1 = T0
lda T0
sta T1
jmp bounded_text_wrap_L43
bounded_text_wrap_L42:
bounded_text_wrap_L43:
; T0 = T0 + 1
lda T0
clc
adc #1
sta T0
jmp bounded_text_wrap_L31
bounded_text_wrap_L33:
; T2 = T0
lda T0
sta T2
; if T0 < COUNT:
lda T0
cmp COUNT
bcc bounded_text_wrap_L49
bounded_text_wrap_L50:
jmp bounded_text_wrap_L47
bounded_text_wrap_L49:
jmp bounded_text_wrap_L46
bounded_text_wrap_L46:
; T3 = C[T0]
ldx T0
lda C0,x
sta T3
; if T3 != 32:
lda T3
cmp #32
bne bounded_text_wrap_L54
bounded_text_wrap_L55:
jmp bounded_text_wrap_L52
bounded_text_wrap_L54:
jmp bounded_text_wrap_L51
bounded_text_wrap_L51:
; if T1 != 255:
lda T1
cmp #255
bne bounded_text_wrap_L59
bounded_text_wrap_L60:
jmp bounded_text_wrap_L57
bounded_text_wrap_L59:
jmp bounded_text_wrap_L56
bounded_text_wrap_L56:
; T2 = T1
lda T1
sta T2
jmp bounded_text_wrap_L58
bounded_text_wrap_L57:
bounded_text_wrap_L58:
jmp bounded_text_wrap_L53
bounded_text_wrap_L52:
bounded_text_wrap_L53:
jmp bounded_text_wrap_L48
bounded_text_wrap_L47:
bounded_text_wrap_L48:
; LENGTH = T2 - START
lda T2
sec
sbc START
sta LENGTH
; NEXT = T2
lda T2
sta NEXT
; while NEXT < COUNT:
bounded_text_wrap_L61:
lda NEXT
cmp COUNT
bcc bounded_text_wrap_L64
bounded_text_wrap_L65:
jmp bounded_text_wrap_L63
bounded_text_wrap_L64:
jmp bounded_text_wrap_L62
bounded_text_wrap_L62:
; T3 = C[NEXT]
ldx NEXT
lda C0,x
sta T3
; if T3 != 32:
lda T3
cmp #32
bne bounded_text_wrap_L69
bounded_text_wrap_L70:
jmp bounded_text_wrap_L67
bounded_text_wrap_L69:
jmp bounded_text_wrap_L66
bounded_text_wrap_L66:
; break
jmp bounded_text_wrap_L63
jmp bounded_text_wrap_L68
bounded_text_wrap_L67:
bounded_text_wrap_L68:
; NEXT = NEXT + 1
lda NEXT
clc
adc #1
sta NEXT
jmp bounded_text_wrap_L61
bounded_text_wrap_L63:
rts
component_end:Machine contract
A/X/status may change; Y and decimal mode are preserved. Use CLD for binary arithmetic. A direct JSR 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 diagnostic PRG owns screen, font and colour RAM and disables interrupts; it does not return to BASIC. The reusable helper does none of those things.
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
LENGTH=$80
NEXT=$81
VALID=$82
START=$83
WIDTH=$84
COUNT=$85
C0=$86
C1=$87
C2=$88
C3=$89
C4=$8a
C5=$8b
C6=$8c
C7=$8d
C8=$8e
C9=$8f
C10=$90
C11=$91
C12=$92
C13=$93
C14=$94
C15=$95
T0=$96
T1=$97
T2=$98
T3=$99
STATE=$80
input_ptr=$70
cases_left=$72
ptr=$74
saved_y=$76
OUTPUT=$c000
preview=OUTPUT+1400
ready=OUTPUT+1408
CUSTOM=0
BROKEN=0
COYOTE_STEPS=4
BUFFER_STEPS=4
TEXT_CAP=8
PROPERTY_COUNT=16
STREAM_BYTES=13
CRC_POLY=7
!source "caller.inc"
!source "exercise.inc"
component_start:
!source "component.inc"
!source "data.inc"
!source "cases.inc"
hex_font:
!byte 0,0,0,0,0,0,0,0,0,56,40,40,40,56,0,0,0,16,48,16,16,56,0,0,0,56,8,56,32,56,0,0,0,56,8,56,8,56,0,0,0,40,40,56,8,8,0,0,0,56,32,56,8,56,0,0,0,56,32,56,40,56,0,0,0,56,8,8,8,8,0,0,0,56,40,56,40,56,0,0,0,56,40,56,8,56,0,0,0,16,40,56,40,40,0,0,0,48,40,48,40,48,0,0,0,56,32,32,32,56,0,0,0,48,40,40,40,48,0,0,0,56,32,48,32,56,0,0,0,56,32,48,32,32,0,0
code_end:A client retaining state between calls
preview_rules:
!if CUSTOM {
; COUNT = 8
lda #8
sta COUNT
; WIDTH = 3
lda #3
sta WIDTH
; C0 = 65
lda #65
sta C0
; C1 = 66
lda #66
sta C1
; C2 = 67
lda #67
sta C2
; C3 = 68
lda #68
sta C3
; C4 = 69
lda #69
sta C4
; C5 = 70
lda #70
sta C5
; C6 = 71
lda #71
sta C6
; C7 = 72
lda #72
sta C7
!if BROKEN {
; WIDTH = 1
lda #1
sta WIDTH
}
jsr checked_step
jsr emit_state
rts
} else {
; COUNT = 11
lda #11
sta COUNT
; WIDTH = 7
lda #7
sta WIDTH
; C0 = 72
lda #72
sta C0
; C1 = 69
lda #69
sta C1
; C2 = 76
lda #76
sta C2
; C3 = 76
lda #76
sta C3
; C4 = 79
lda #79
sta C4
; C5 = 32
lda #32
sta C5
; C6 = 87
lda #87
sta C6
; C7 = 79
lda #79
sta C7
; C8 = 82
lda #82
sta C8
; C9 = 76
lda #76
sta C9
; C10 = 68
lda #68
sta C10
!if BROKEN {
; WIDTH = 1
lda #1
sta WIDTH
}
jsr checked_step
jsr emit_state
}
rtsCustomise 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
LENGTH=$80
NEXT=$81
VALID=$82
START=$83
WIDTH=$84
COUNT=$85
C0=$86
C1=$87
C2=$88
C3=$89
C4=$8a
C5=$8b
C6=$8c
C7=$8d
C8=$8e
C9=$8f
C10=$90
C11=$91
C12=$92
C13=$93
C14=$94
C15=$95
T0=$96
T1=$97
T2=$98
T3=$99
STATE=$80
input_ptr=$70
cases_left=$72
ptr=$74
saved_y=$76
OUTPUT=$c000
preview=OUTPUT+1400
ready=OUTPUT+1408
CUSTOM=1
BROKEN=0
COYOTE_STEPS=2
BUFFER_STEPS=2
TEXT_CAP=4
PROPERTY_COUNT=16
STREAM_BYTES=13
CRC_POLY=49
!source "caller.inc"
!source "exercise.inc"
component_start:
!source "component.inc"
!source "data.inc"
!source "cases.inc"
hex_font:
!byte 0,0,0,0,0,0,0,0,0,56,40,40,40,56,0,0,0,16,48,16,16,56,0,0,0,56,8,56,32,56,0,0,0,56,8,56,8,56,0,0,0,40,40,56,8,8,0,0,0,56,32,56,8,56,0,0,0,56,32,56,40,56,0,0,0,56,8,8,8,8,0,0,0,56,40,56,40,56,0,0,0,56,40,56,8,56,0,0,0,16,40,56,40,40,0,0,0,48,40,48,40,48,0,0,0,56,32,32,32,56,0,0,0,48,40,40,40,48,0,0,0,56,32,48,32,56,0,0,0,56,32,48,32,32,0,0
code_end:The 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
LENGTH=$80
NEXT=$81
VALID=$82
START=$83
WIDTH=$84
COUNT=$85
C0=$86
C1=$87
C2=$88
C3=$89
C4=$8a
C5=$8b
C6=$8c
C7=$8d
C8=$8e
C9=$8f
C10=$90
C11=$91
C12=$92
C13=$93
C14=$94
C15=$95
T0=$96
T1=$97
T2=$98
T3=$99
STATE=$80
input_ptr=$70
cases_left=$72
ptr=$74
saved_y=$76
OUTPUT=$c000
preview=OUTPUT+1400
ready=OUTPUT+1408
CUSTOM=0
BROKEN=1
COYOTE_STEPS=4
BUFFER_STEPS=4
TEXT_CAP=8
PROPERTY_COUNT=16
STREAM_BYTES=13
CRC_POLY=7
!source "caller.inc"
!source "exercise.inc"
component_start:
!source "component.inc"
!source "data.inc"
!source "cases.inc"
hex_font:
!byte 0,0,0,0,0,0,0,0,0,56,40,40,40,56,0,0,0,16,48,16,16,56,0,0,0,56,8,56,32,56,0,0,0,56,8,56,8,56,0,0,0,40,40,56,8,8,0,0,0,56,32,56,8,56,0,0,0,56,32,56,40,56,0,0,0,56,8,8,8,8,0,0,0,56,40,56,40,56,0,0,0,56,40,56,8,56,0,0,0,16,40,56,40,40,0,0,0,48,40,48,40,48,0,0,0,56,32,32,32,56,0,0,0,48,40,40,40,48,0,0,0,56,32,48,32,56,0,0,0,56,32,48,32,32,0,0
code_end:
Cost and alternatives
The measured helper extent is 388 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 PAL stock-ROM PRG, 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.