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:
ld a,0
ld (REVEALED),a
ld (TICKS),a
ld (DONE),a
ld (ADDED),a
ld (VALID),a
ld (LENGTH),a
ld (INTERVAL),a
ld (SKIP),a
ret
typewriter_text_step:
; DONE = 0
ld a,0
ld (DONE),a
; ADDED = 0
ld a,0
ld (ADDED),a
; VALID = 0
ld a,0
ld (VALID),a
; if INTERVAL == 0:
ld a,(INTERVAL)
cp 0
jp z,typewriter_text_L1
jp typewriter_text_L2
typewriter_text_L1:
; return
ret
jp typewriter_text_L3
typewriter_text_L2:
typewriter_text_L3:
; if TICKS >= INTERVAL:
ld a,(TICKS)
ld b,a
ld a,(INTERVAL)
ld c,a
ld a,b
cp c
jp nc,typewriter_text_L4
jp typewriter_text_L5
typewriter_text_L4:
; return
ret
jp typewriter_text_L6
typewriter_text_L5:
typewriter_text_L6:
; if REVEALED > LENGTH:
ld a,(REVEALED)
ld b,a
ld a,(LENGTH)
ld c,a
ld a,b
cp c
jp z,typewriter_text_L8
jp nc,typewriter_text_L7
jp typewriter_text_L8
typewriter_text_L7:
; return
ret
jp typewriter_text_L9
typewriter_text_L8:
typewriter_text_L9:
; VALID = 1
ld a,1
ld (VALID),a
; if SKIP != 0:
ld a,(SKIP)
cp 0
jp nz,typewriter_text_L10
jp typewriter_text_L11
typewriter_text_L10:
; ADDED = LENGTH - REVEALED
ld a,(LENGTH)
ld b,a
ld a,(REVEALED)
ld c,a
ld a,b
sub c
ld (ADDED),a
; REVEALED = LENGTH
ld a,(LENGTH)
ld (REVEALED),a
; TICKS = 0
ld a,0
ld (TICKS),a
jp typewriter_text_L12
typewriter_text_L11:
; if REVEALED < LENGTH:
ld a,(REVEALED)
ld b,a
ld a,(LENGTH)
ld c,a
ld a,b
cp c
jp c,typewriter_text_L13
jp typewriter_text_L14
typewriter_text_L13:
; TICKS = TICKS + 1
ld a,(TICKS)
add a,1
ld (TICKS),a
; if TICKS == INTERVAL:
ld a,(TICKS)
ld b,a
ld a,(INTERVAL)
ld c,a
ld a,b
cp c
jp z,typewriter_text_L16
jp typewriter_text_L17
typewriter_text_L16:
; TICKS = 0
ld a,0
ld (TICKS),a
; REVEALED = REVEALED + 1
ld a,(REVEALED)
add a,1
ld (REVEALED),a
; ADDED = 1
ld a,1
ld (ADDED),a
jp typewriter_text_L18
typewriter_text_L17:
typewriter_text_L18:
jp typewriter_text_L15
typewriter_text_L14:
typewriter_text_L15:
typewriter_text_L12:
; if REVEALED == LENGTH:
ld a,(REVEALED)
ld b,a
ld a,(LENGTH)
ld c,a
ld a,b
cp c
jp z,typewriter_text_L19
jp typewriter_text_L20
typewriter_text_L19:
; DONE = 1
ld a,1
ld (DONE),a
jp typewriter_text_L21
typewriter_text_L20:
typewriter_text_L21:
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:
REVEALED: defb 0
TICKS: defb 0
DONE: defb 0
ADDED: defb 0
VALID: defb 0
LENGTH: defb 0
INTERVAL: defb 0
SKIP: 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
; LENGTH = 3
ld a,3
ld (LENGTH),a
; INTERVAL = 1
ld a,1
ld (INTERVAL),a
if BROKEN
; TICKS = 0
ld a,0
ld (TICKS),a
endif
call checked_step
call emit_state
; SKIP = 1
ld a,1
ld (SKIP),a
if BROKEN
; TICKS = 0
ld a,0
ld (TICKS),a
endif
call checked_step
call emit_state
; SKIP = 0
ld a,0
ld (SKIP),a
if BROKEN
; TICKS = 0
ld a,0
ld (TICKS),a
endif
call checked_step
call emit_state
if BROKEN
; TICKS = 0
ld a,0
ld (TICKS),a
endif
call checked_step
call emit_state
ret
else
; LENGTH = 4
ld a,4
ld (LENGTH),a
; INTERVAL = 2
ld a,2
ld (INTERVAL),a
if BROKEN
; TICKS = 0
ld a,0
ld (TICKS),a
endif
call checked_step
call emit_state
if BROKEN
; TICKS = 0
ld a,0
ld (TICKS),a
endif
call checked_step
call emit_state
if BROKEN
; TICKS = 0
ld a,0
ld (TICKS),a
endif
call checked_step
call emit_state
if BROKEN
; TICKS = 0
ld a,0
ld (TICKS),a
endif
call checked_step
call emit_state
endif
retCustomise 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
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:
REVEALED: defb 0
TICKS: defb 0
DONE: defb 0
ADDED: defb 0
VALID: defb 0
LENGTH: defb 0
INTERVAL: defb 0
SKIP: 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 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
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:
REVEALED: defb 0
TICKS: defb 0
DONE: defb 0
ADDED: defb 0
VALID: defb 0
LENGTH: defb 0
INTERVAL: defb 0
SKIP: 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 232 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 48K PAL ROM tape load and USR, 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.