A line of dialogue can appear a character at a time, with a button to show the rest immediately.
Use it in a project
LENGTH is 0–255 bytes. REVEALED must not exceed it. INTERVAL must be nonzero and TICKS below it. Each service consumes one tick; on expiry reveal one byte. Nonzero SKIP reveals the whole counted string immediately and clears TICKS. DONE reflects REVEALED==LENGTH and ADDED reports new bytes this service. Completion is stable while the same LENGTH is retained. Invalid state clears transient outputs and leaves REVEALED/TICKS unchanged.
Include component.inc once (MOS callers include the shared 6502 implementation through it). Reserve the public fields in this exact order: REVEALED, TICKS, DONE, ADDED, VALID, LENGTH, INTERVAL, SKIP. They occupy 8 contiguous bytes. Also reserve the private workspace: none (0 bytes). Array names in the explanation refer to the numbered consecutive bindings, such as C0, C1 and so on.
Call typewriter_text_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 typewriter_text_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 | REVEALED and TICKS |
| Supply before service | LENGTH, INTERVAL and SKIP |
| Read after service | REVEALED, TICKS, DONE, ADDED and VALID |
The timer changes the exposed prefix length, while the renderer owns how that prefix looks. DONE is derived from the prefix length so a completed line stays complete without another animation state.
Reusable initialisation and service
; SPDX-License-Identifier: MIT
; Reveal Counted Text Over Time. See README.md for the byte interface and ownership.
; init clears the public fields and private workspace; set inputs afterwards.
typewriter_text_init:
lda #0
sta REVEALED
sta TICKS
sta DONE
sta ADDED
sta VALID
sta LENGTH
sta INTERVAL
sta SKIP
rts
typewriter_text_step:
; DONE = 0
lda #0
sta DONE
; ADDED = 0
lda #0
sta ADDED
; VALID = 0
lda #0
sta VALID
; if INTERVAL == 0:
lda INTERVAL
cmp #0
beq typewriter_text_L4
typewriter_text_L5:
jmp typewriter_text_L2
typewriter_text_L4:
jmp typewriter_text_L1
typewriter_text_L1:
; return
rts
jmp typewriter_text_L3
typewriter_text_L2:
typewriter_text_L3:
; if TICKS >= INTERVAL:
lda TICKS
cmp INTERVAL
bcs typewriter_text_L9
typewriter_text_L10:
jmp typewriter_text_L7
typewriter_text_L9:
jmp typewriter_text_L6
typewriter_text_L6:
; return
rts
jmp typewriter_text_L8
typewriter_text_L7:
typewriter_text_L8:
; if REVEALED > LENGTH:
lda REVEALED
cmp LENGTH
beq typewriter_text_L15
bcs typewriter_text_L14
typewriter_text_L15:
jmp typewriter_text_L12
typewriter_text_L14:
jmp typewriter_text_L11
typewriter_text_L11:
; return
rts
jmp typewriter_text_L13
typewriter_text_L12:
typewriter_text_L13:
; VALID = 1
lda #1
sta VALID
; if SKIP != 0:
lda SKIP
cmp #0
bne typewriter_text_L19
typewriter_text_L20:
jmp typewriter_text_L17
typewriter_text_L19:
jmp typewriter_text_L16
typewriter_text_L16:
; ADDED = LENGTH - REVEALED
lda LENGTH
sec
sbc REVEALED
sta ADDED
; REVEALED = LENGTH
lda LENGTH
sta REVEALED
; TICKS = 0
lda #0
sta TICKS
jmp typewriter_text_L18
typewriter_text_L17:
; if REVEALED < LENGTH:
lda REVEALED
cmp LENGTH
bcc typewriter_text_L24
typewriter_text_L25:
jmp typewriter_text_L22
typewriter_text_L24:
jmp typewriter_text_L21
typewriter_text_L21:
; TICKS = TICKS + 1
lda TICKS
clc
adc #1
sta TICKS
; if TICKS == INTERVAL:
lda TICKS
cmp INTERVAL
beq typewriter_text_L29
typewriter_text_L30:
jmp typewriter_text_L27
typewriter_text_L29:
jmp typewriter_text_L26
typewriter_text_L26:
; TICKS = 0
lda #0
sta TICKS
; REVEALED = REVEALED + 1
lda REVEALED
clc
adc #1
sta REVEALED
; ADDED = 1
lda #1
sta ADDED
jmp typewriter_text_L28
typewriter_text_L27:
typewriter_text_L28:
jmp typewriter_text_L23
typewriter_text_L22:
typewriter_text_L23:
typewriter_text_L18:
; if REVEALED == LENGTH:
lda REVEALED
cmp LENGTH
beq typewriter_text_L34
typewriter_text_L35:
jmp typewriter_text_L32
typewriter_text_L34:
jmp typewriter_text_L31
typewriter_text_L31:
; DONE = 1
lda #1
sta DONE
jmp typewriter_text_L33
typewriter_text_L32:
typewriter_text_L33:
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
REVEALED=$80
TICKS=$81
DONE=$82
ADDED=$83
VALID=$84
LENGTH=$85
INTERVAL=$86
SKIP=$87
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 {
; LENGTH = 3
lda #3
sta LENGTH
; INTERVAL = 1
lda #1
sta INTERVAL
!if BROKEN {
; TICKS = 0
lda #0
sta TICKS
}
jsr checked_step
jsr emit_state
; SKIP = 1
lda #1
sta SKIP
!if BROKEN {
; TICKS = 0
lda #0
sta TICKS
}
jsr checked_step
jsr emit_state
; SKIP = 0
lda #0
sta SKIP
!if BROKEN {
; TICKS = 0
lda #0
sta TICKS
}
jsr checked_step
jsr emit_state
!if BROKEN {
; TICKS = 0
lda #0
sta TICKS
}
jsr checked_step
jsr emit_state
rts
} else {
; LENGTH = 4
lda #4
sta LENGTH
; INTERVAL = 2
lda #2
sta INTERVAL
!if BROKEN {
; TICKS = 0
lda #0
sta TICKS
}
jsr checked_step
jsr emit_state
!if BROKEN {
; TICKS = 0
lda #0
sta TICKS
}
jsr checked_step
jsr emit_state
!if BROKEN {
; TICKS = 0
lda #0
sta TICKS
}
jsr checked_step
jsr emit_state
!if BROKEN {
; TICKS = 0
lda #0
sta TICKS
}
jsr checked_step
jsr emit_state
}
rtsCustomise it
Reveal one byte per service and observe that further calls after completion add nothing. 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
REVEALED=$80
TICKS=$81
DONE=$82
ADDED=$83
VALID=$84
LENGTH=$85
INTERVAL=$86
SKIP=$87
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 00 01 00 01 03 01 00. The full client trace in the source explains how it gets there.
Mistake and repair
The wrong client clears the interval counter every service. With interval two the line never reveals its first character. Retain TICKS and reset only at the intended message boundary.
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
REVEALED=$80
TICKS=$81
DONE=$82
ADDED=$83
VALID=$84
LENGTH=$85
INTERVAL=$86
SKIP=$87
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 187 code bytes; public state is 8 bytes and private scratch is 0 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.
Constant work per tick; skip changes a count rather than looping over every byte. Drawing an entire prefix on each call can still be expensive. Connect a byte span or dirty-text renderer according to the machine.
Counts refer to bytes in a caller-owned stable string, not UTF-8 characters, markup tokens or proportional glyphs. Keep layout separate and draw the newly exposed span; control codes need a token-aware variant. Changing LENGTH is not a reset protocol: initialise a new message deliberately.
Execution evidence
Four variants run on PAL stock-ROM PRG, using Asm198x 0.0.72, Emu198x 0.44.2. Each executes 64 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.
Where it fits
Bounded text spans: Lay out complete counted text before revealing it so the line does not reflow as characters appear.