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:
move.b #0,d0
move.b d0,LENGTH
move.b d0,NEXT
move.b d0,VALID
move.b d0,START
move.b d0,WIDTH
move.b d0,COUNT
move.b d0,C0
move.b d0,C1
move.b d0,C2
move.b d0,C3
move.b d0,C4
move.b d0,C5
move.b d0,C6
move.b d0,C7
move.b d0,C8
move.b d0,C9
move.b d0,C10
move.b d0,C11
move.b d0,C12
move.b d0,C13
move.b d0,C14
move.b d0,C15
move.b d0,T0
move.b d0,T1
move.b d0,T2
move.b d0,T3
rts
bounded_text_wrap_step:
; LENGTH = 0
move.b #0,d0
move.b d0,LENGTH
; NEXT = START
move.b START,d0
move.b d0,NEXT
; VALID = 0
move.b #0,d0
move.b d0,VALID
; if COUNT > 16:
move.b COUNT,d0
cmp.b #16,d0
bhi bounded_text_wrap_L1
jmp bounded_text_wrap_L2
bounded_text_wrap_L1:
; return
rts
jmp bounded_text_wrap_L3
bounded_text_wrap_L2:
bounded_text_wrap_L3:
; if START > COUNT:
move.b START,d0
move.b COUNT,d1
cmp.b d1,d0
bhi bounded_text_wrap_L4
jmp bounded_text_wrap_L5
bounded_text_wrap_L4:
; return
rts
jmp bounded_text_wrap_L6
bounded_text_wrap_L5:
bounded_text_wrap_L6:
; if WIDTH == 0:
move.b WIDTH,d0
cmp.b #0,d0
beq bounded_text_wrap_L7
jmp bounded_text_wrap_L8
bounded_text_wrap_L7:
; return
rts
jmp bounded_text_wrap_L9
bounded_text_wrap_L8:
bounded_text_wrap_L9:
; if WIDTH > 16:
move.b WIDTH,d0
cmp.b #16,d0
bhi bounded_text_wrap_L10
jmp bounded_text_wrap_L11
bounded_text_wrap_L10:
; return
rts
jmp bounded_text_wrap_L12
bounded_text_wrap_L11:
bounded_text_wrap_L12:
; VALID = 1
move.b #1,d0
move.b d0,VALID
; while START < COUNT:
bounded_text_wrap_L13:
move.b START,d0
move.b COUNT,d1
cmp.b d1,d0
bcs bounded_text_wrap_L14
jmp bounded_text_wrap_L15
bounded_text_wrap_L14:
; T0 = C[START]
moveq #0,d1
move.b START,d1
lea C0,a0
move.b 0(a0,d1.w),d0
move.b d0,T0
; if T0 != 32:
move.b T0,d0
cmp.b #32,d0
bne bounded_text_wrap_L16
jmp bounded_text_wrap_L17
bounded_text_wrap_L16:
; break
jmp bounded_text_wrap_L15
jmp bounded_text_wrap_L18
bounded_text_wrap_L17:
bounded_text_wrap_L18:
; START = START + 1
move.b START,d0
add.b #1,d0
move.b d0,START
jmp bounded_text_wrap_L13
bounded_text_wrap_L15:
; NEXT = START
move.b START,d0
move.b d0,NEXT
; T0 = START
move.b START,d0
move.b d0,T0
; T1 = 255
move.b #255,d0
move.b d0,T1
; while T0 < COUNT:
bounded_text_wrap_L19:
move.b T0,d0
move.b COUNT,d1
cmp.b d1,d0
bcs bounded_text_wrap_L20
jmp bounded_text_wrap_L21
bounded_text_wrap_L20:
; T2 = T0 - START
move.b T0,d0
move.b START,d1
sub.b d1,d0
move.b d0,T2
; if T2 == WIDTH:
move.b T2,d0
move.b WIDTH,d1
cmp.b d1,d0
beq bounded_text_wrap_L22
jmp bounded_text_wrap_L23
bounded_text_wrap_L22:
; break
jmp bounded_text_wrap_L21
jmp bounded_text_wrap_L24
bounded_text_wrap_L23:
bounded_text_wrap_L24:
; T2 = C[T0]
moveq #0,d1
move.b T0,d1
lea C0,a0
move.b 0(a0,d1.w),d0
move.b d0,T2
; if T2 == 32:
move.b T2,d0
cmp.b #32,d0
beq bounded_text_wrap_L25
jmp bounded_text_wrap_L26
bounded_text_wrap_L25:
; T1 = T0
move.b T0,d0
move.b d0,T1
jmp bounded_text_wrap_L27
bounded_text_wrap_L26:
bounded_text_wrap_L27:
; T0 = T0 + 1
move.b T0,d0
add.b #1,d0
move.b d0,T0
jmp bounded_text_wrap_L19
bounded_text_wrap_L21:
; T2 = T0
move.b T0,d0
move.b d0,T2
; if T0 < COUNT:
move.b T0,d0
move.b COUNT,d1
cmp.b d1,d0
bcs bounded_text_wrap_L28
jmp bounded_text_wrap_L29
bounded_text_wrap_L28:
; T3 = C[T0]
moveq #0,d1
move.b T0,d1
lea C0,a0
move.b 0(a0,d1.w),d0
move.b d0,T3
; if T3 != 32:
move.b T3,d0
cmp.b #32,d0
bne bounded_text_wrap_L31
jmp bounded_text_wrap_L32
bounded_text_wrap_L31:
; if T1 != 255:
move.b T1,d0
cmp.b #255,d0
bne bounded_text_wrap_L34
jmp bounded_text_wrap_L35
bounded_text_wrap_L34:
; T2 = T1
move.b T1,d0
move.b d0,T2
jmp bounded_text_wrap_L36
bounded_text_wrap_L35:
bounded_text_wrap_L36:
jmp bounded_text_wrap_L33
bounded_text_wrap_L32:
bounded_text_wrap_L33:
jmp bounded_text_wrap_L30
bounded_text_wrap_L29:
bounded_text_wrap_L30:
; LENGTH = T2 - START
move.b T2,d0
move.b START,d1
sub.b d1,d0
move.b d0,LENGTH
; NEXT = T2
move.b T2,d0
move.b d0,NEXT
; while NEXT < COUNT:
bounded_text_wrap_L37:
move.b NEXT,d0
move.b COUNT,d1
cmp.b d1,d0
bcs bounded_text_wrap_L38
jmp bounded_text_wrap_L39
bounded_text_wrap_L38:
; T3 = C[NEXT]
moveq #0,d1
move.b NEXT,d1
lea C0,a0
move.b 0(a0,d1.w),d0
move.b d0,T3
; if T3 != 32:
move.b T3,d0
cmp.b #32,d0
bne bounded_text_wrap_L40
jmp bounded_text_wrap_L41
bounded_text_wrap_L40:
; break
jmp bounded_text_wrap_L39
jmp bounded_text_wrap_L42
bounded_text_wrap_L41:
bounded_text_wrap_L42:
; NEXT = NEXT + 1
move.b NEXT,d0
add.b #1,d0
move.b d0,NEXT
jmp bounded_text_wrap_L37
bounded_text_wrap_L39:
rts
component_end:Machine contract
D0–D3/A0–A1/CCR may change; D4–D7/A2–A6 are preserved. A direct BSR/JSR needs four 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 A500 OCS diagnostic cold-boots an OFS disk, owns the Copper and bitplane and disables interrupt/DMA sources. The helper does not take over hardware or allocate Chip RAM.
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 and BUILD198X=/path/to/build198x. The Makefile produces normal, custom, deliberately wrong and repaired programs. Tools must already be installed; the sample downloads nothing.
Bindings and include order
section code,code_c
CUSTOM = 0
BROKEN = 0
COYOTE_STEPS = 4
BUFFER_STEPS = 4
TEXT_CAP = 8
PROPERTY_COUNT = 16
STREAM_BYTES = 13
CRC_POLY = 7
include "caller.inc"
include "exercise.inc"
component_start:
include "component.inc"
even
include "data.inc"
include "cases.inc"
even
include "display.inc"
include "../../framework/turn-deadline/font.inc"
code_end:
STATE:
LENGTH: dc.b 0
NEXT: dc.b 0
VALID: dc.b 0
START: dc.b 0
WIDTH: dc.b 0
COUNT: dc.b 0
C0: dc.b 0
C1: dc.b 0
C2: dc.b 0
C3: dc.b 0
C4: dc.b 0
C5: dc.b 0
C6: dc.b 0
C7: dc.b 0
C8: dc.b 0
C9: dc.b 0
C10: dc.b 0
C11: dc.b 0
C12: dc.b 0
C13: dc.b 0
C14: dc.b 0
C15: dc.b 0
T0: dc.b 0
T1: dc.b 0
T2: dc.b 0
T3: dc.b 0
even
ready: dc.w 0
preview: dcb.b 8,0
OUTPUT: dcb.b 1300,0
even
signature: dc.b "198XTOOLKIT0000",0
even
copperlist:
dc.w $08e,$2c81,$090,$2cc1,$092,$0038,$094,$00d0
dc.w $100,$1200,$102,0,$104,0,$108,0,$10a,0
dc.w $180,$012,$182,$fff
copbpl:
dc.w $0e0,0,$0e2,0
dc.w $ffff,$fffe
plane: dcb.b 10240,0A client retaining state between calls
preview_rules:
ifne CUSTOM
; COUNT = 8
move.b #8,d0
move.b d0,COUNT
; WIDTH = 3
move.b #3,d0
move.b d0,WIDTH
; C0 = 65
move.b #65,d0
move.b d0,C0
; C1 = 66
move.b #66,d0
move.b d0,C1
; C2 = 67
move.b #67,d0
move.b d0,C2
; C3 = 68
move.b #68,d0
move.b d0,C3
; C4 = 69
move.b #69,d0
move.b d0,C4
; C5 = 70
move.b #70,d0
move.b d0,C5
; C6 = 71
move.b #71,d0
move.b d0,C6
; C7 = 72
move.b #72,d0
move.b d0,C7
ifne BROKEN
; WIDTH = 1
move.b #1,d0
move.b d0,WIDTH
endif
bsr checked_step
bsr emit_state
rts
else
; COUNT = 11
move.b #11,d0
move.b d0,COUNT
; WIDTH = 7
move.b #7,d0
move.b d0,WIDTH
; C0 = 72
move.b #72,d0
move.b d0,C0
; C1 = 69
move.b #69,d0
move.b d0,C1
; C2 = 76
move.b #76,d0
move.b d0,C2
; C3 = 76
move.b #76,d0
move.b d0,C3
; C4 = 79
move.b #79,d0
move.b d0,C4
; C5 = 32
move.b #32,d0
move.b d0,C5
; C6 = 87
move.b #87,d0
move.b d0,C6
; C7 = 79
move.b #79,d0
move.b d0,C7
; C8 = 82
move.b #82,d0
move.b d0,C8
; C9 = 76
move.b #76,d0
move.b d0,C9
; C10 = 68
move.b #68,d0
move.b d0,C10
ifne BROKEN
; WIDTH = 1
move.b #1,d0
move.b d0,WIDTH
endif
bsr checked_step
bsr emit_state
endif
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
section code,code_c
CUSTOM = 1
BROKEN = 0
COYOTE_STEPS = 2
BUFFER_STEPS = 2
TEXT_CAP = 4
PROPERTY_COUNT = 16
STREAM_BYTES = 13
CRC_POLY = 49
include "caller.inc"
include "exercise.inc"
component_start:
include "component.inc"
even
include "data.inc"
include "cases.inc"
even
include "display.inc"
include "../../framework/turn-deadline/font.inc"
code_end:
STATE:
LENGTH: dc.b 0
NEXT: dc.b 0
VALID: dc.b 0
START: dc.b 0
WIDTH: dc.b 0
COUNT: dc.b 0
C0: dc.b 0
C1: dc.b 0
C2: dc.b 0
C3: dc.b 0
C4: dc.b 0
C5: dc.b 0
C6: dc.b 0
C7: dc.b 0
C8: dc.b 0
C9: dc.b 0
C10: dc.b 0
C11: dc.b 0
C12: dc.b 0
C13: dc.b 0
C14: dc.b 0
C15: dc.b 0
T0: dc.b 0
T1: dc.b 0
T2: dc.b 0
T3: dc.b 0
even
ready: dc.w 0
preview: dcb.b 8,0
OUTPUT: dcb.b 1300,0
even
signature: dc.b "198XTOOLKIT0000",0
even
copperlist:
dc.w $08e,$2c81,$090,$2cc1,$092,$0038,$094,$00d0
dc.w $100,$1200,$102,0,$104,0,$108,0,$10a,0
dc.w $180,$012,$182,$fff
copbpl:
dc.w $0e0,0,$0e2,0
dc.w $ffff,$fffe
plane: dcb.b 10240,0The 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
section code,code_c
CUSTOM = 0
BROKEN = 1
COYOTE_STEPS = 4
BUFFER_STEPS = 4
TEXT_CAP = 8
PROPERTY_COUNT = 16
STREAM_BYTES = 13
CRC_POLY = 7
include "caller.inc"
include "exercise.inc"
component_start:
include "component.inc"
even
include "data.inc"
include "cases.inc"
even
include "display.inc"
include "../../framework/turn-deadline/font.inc"
code_end:
STATE:
LENGTH: dc.b 0
NEXT: dc.b 0
VALID: dc.b 0
START: dc.b 0
WIDTH: dc.b 0
COUNT: dc.b 0
C0: dc.b 0
C1: dc.b 0
C2: dc.b 0
C3: dc.b 0
C4: dc.b 0
C5: dc.b 0
C6: dc.b 0
C7: dc.b 0
C8: dc.b 0
C9: dc.b 0
C10: dc.b 0
C11: dc.b 0
C12: dc.b 0
C13: dc.b 0
C14: dc.b 0
C15: dc.b 0
T0: dc.b 0
T1: dc.b 0
T2: dc.b 0
T3: dc.b 0
even
ready: dc.w 0
preview: dcb.b 8,0
OUTPUT: dcb.b 1300,0
even
signature: dc.b "198XTOOLKIT0000",0
even
copperlist:
dc.w $08e,$2c81,$090,$2cc1,$092,$0038,$094,$00d0
dc.w $100,$1200,$102,0,$104,0,$108,0,$10a,0
dc.w $180,$012,$182,$fff
copbpl:
dc.w $0e0,0,$0e2,0
dc.w $ffff,$fffe
plane: dcb.b 10240,0
Cost and alternatives
The measured helper extent is 704 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 A500 OCS PAL, Kickstart1.3 OFS cold boot, using Asm198x 0.0.72, Emu198x 0.44.2 and Build198x 0.2.9. 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.