A dialogue option can depend on what the player knows and change what happens next.
Use it in a project
REQUIRED and FORBIDDEN are disjoint masks. A choice is AVAILABLE when every required flag is set and no forbidden flag is set. ACCEPT nonzero commits only an available choice: clear CLEAR_MASK then set SET_MASK, so set wins overlap. TAKEN pulses on commit. Invalid gating masks leave FLAGS unchanged and clear result flags.
Include component.inc once (MOS callers include the shared 6502 implementation through it). Reserve the public fields in this exact order: FLAGS, REQUIRED, FORBIDDEN, SET_MASK, CLEAR_MASK, ACCEPT, AVAILABLE, TAKEN, VALID. They occupy 9 contiguous bytes. Also reserve the private workspace: T0 (1 bytes). Array names in the explanation refer to the numbered consecutive bindings, such as C0, C1 and so on.
Call dialogue_gates_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 dialogue_gates_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 | FLAGS |
| Supply before service | REQUIRED, FORBIDDEN, SET_MASK, CLEAR_MASK and ACCEPT |
| Read after service | AVAILABLE, TAKEN and VALID |
Availability is a pure condition until ACCEPT is supplied. Committing clears and sets story bits in a defined order, so the UI can show options without accidentally taking them.
Reusable initialisation and service
; SPDX-License-Identifier: MIT
; Gate a Dialogue Choice with Flags. See README.md for the byte interface and ownership.
; init clears the public fields and private workspace; set inputs afterwards.
dialogue_gates_init:
ld a,0
ld (FLAGS),a
ld (REQUIRED),a
ld (FORBIDDEN),a
ld (SET_MASK),a
ld (CLEAR_MASK),a
ld (ACCEPT),a
ld (AVAILABLE),a
ld (TAKEN),a
ld (VALID),a
ld (T0),a
ret
dialogue_gates_step:
; AVAILABLE = 0
ld a,0
ld (AVAILABLE),a
; TAKEN = 0
ld a,0
ld (TAKEN),a
; VALID = 0
ld a,0
ld (VALID),a
; T0 = REQUIRED & FORBIDDEN
ld a,(REQUIRED)
ld b,a
ld a,(FORBIDDEN)
ld c,a
ld a,b
and c
ld (T0),a
; if T0 != 0:
ld a,(T0)
cp 0
jp nz,dialogue_gates_L1
jp dialogue_gates_L2
dialogue_gates_L1:
; return
ret
jp dialogue_gates_L3
dialogue_gates_L2:
dialogue_gates_L3:
; VALID = 1
ld a,1
ld (VALID),a
; T0 = FLAGS & REQUIRED
ld a,(FLAGS)
ld b,a
ld a,(REQUIRED)
ld c,a
ld a,b
and c
ld (T0),a
; if T0 != REQUIRED:
ld a,(T0)
ld b,a
ld a,(REQUIRED)
ld c,a
ld a,b
cp c
jp nz,dialogue_gates_L4
jp dialogue_gates_L5
dialogue_gates_L4:
; return
ret
jp dialogue_gates_L6
dialogue_gates_L5:
dialogue_gates_L6:
; T0 = FLAGS & FORBIDDEN
ld a,(FLAGS)
ld b,a
ld a,(FORBIDDEN)
ld c,a
ld a,b
and c
ld (T0),a
; if T0 != 0:
ld a,(T0)
cp 0
jp nz,dialogue_gates_L7
jp dialogue_gates_L8
dialogue_gates_L7:
; return
ret
jp dialogue_gates_L9
dialogue_gates_L8:
dialogue_gates_L9:
; AVAILABLE = 1
ld a,1
ld (AVAILABLE),a
; if ACCEPT == 0:
ld a,(ACCEPT)
cp 0
jp z,dialogue_gates_L10
jp dialogue_gates_L11
dialogue_gates_L10:
; return
ret
jp dialogue_gates_L12
dialogue_gates_L11:
dialogue_gates_L12:
; T0 = CLEAR_MASK ^ 255
ld a,(CLEAR_MASK)
xor 255
ld (T0),a
; FLAGS = FLAGS & T0
ld a,(FLAGS)
ld b,a
ld a,(T0)
ld c,a
ld a,b
and c
ld (FLAGS),a
; FLAGS = FLAGS | SET_MASK
ld a,(FLAGS)
ld b,a
ld a,(SET_MASK)
ld c,a
ld a,b
or c
ld (FLAGS),a
; TAKEN = 1
ld a,1
ld (TAKEN),a
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. Scope or rename routine and field labels and allocate a separate block for each independent instance. 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:
FLAGS: defb 0
REQUIRED: defb 0
FORBIDDEN: defb 0
SET_MASK: defb 0
CLEAR_MASK: defb 0
ACCEPT: defb 0
AVAILABLE: defb 0
TAKEN: defb 0
VALID: defb 0
T0: 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
; FLAGS = 5
ld a,5
ld (FLAGS),a
; REQUIRED = 5
ld a,5
ld (REQUIRED),a
; FORBIDDEN = 2
ld a,2
ld (FORBIDDEN),a
; SET_MASK = 2
ld a,2
ld (SET_MASK),a
; CLEAR_MASK = 1
ld a,1
ld (CLEAR_MASK),a
; ACCEPT = 0
ld a,0
ld (ACCEPT),a
if BROKEN
; FLAGS = 0
ld a,0
ld (FLAGS),a
endif
call checked_step
call emit_state
; ACCEPT = 1
ld a,1
ld (ACCEPT),a
if BROKEN
; FLAGS = 0
ld a,0
ld (FLAGS),a
endif
call checked_step
call emit_state
if BROKEN
; FLAGS = 0
ld a,0
ld (FLAGS),a
endif
call checked_step
call emit_state
ret
else
; FLAGS = 1
ld a,1
ld (FLAGS),a
; REQUIRED = 1
ld a,1
ld (REQUIRED),a
; FORBIDDEN = 2
ld a,2
ld (FORBIDDEN),a
; SET_MASK = 2
ld a,2
ld (SET_MASK),a
; CLEAR_MASK = 1
ld a,1
ld (CLEAR_MASK),a
; ACCEPT = 0
ld a,0
ld (ACCEPT),a
if BROKEN
; FLAGS = 0
ld a,0
ld (FLAGS),a
endif
call checked_step
call emit_state
; ACCEPT = 1
ld a,1
ld (ACCEPT),a
if BROKEN
; FLAGS = 0
ld a,0
ld (FLAGS),a
endif
call checked_step
call emit_state
if BROKEN
; FLAGS = 0
ld a,0
ld (FLAGS),a
endif
call checked_step
call emit_state
endif
retCustomise it
Preview and accept a choice requiring flags 1 and 4. 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:
FLAGS: defb 0
REQUIRED: defb 0
FORBIDDEN: defb 0
SET_MASK: defb 0
CLEAR_MASK: defb 0
ACCEPT: defb 0
AVAILABLE: defb 0
TAKEN: defb 0
VALID: defb 0
T0: 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 06 05 02 02 01 01 00 00. The full client trace in the source explains how it gets there.
Mistake and repair
The wrong caller clears FLAGS before evaluating a choice. The player loses the knowledge that should make the option available. Preview and commit against the same persistent story state.
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:
FLAGS: defb 0
REQUIRED: defb 0
FORBIDDEN: defb 0
SET_MASK: defb 0
CLEAR_MASK: defb 0
ACCEPT: defb 0
AVAILABLE: defb 0
TAKEN: defb 0
VALID: defb 0
T0: 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 202 code bytes; public state is 9 bytes and private scratch is 1 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 on one flag byte. Larger stories can evaluate several bytes, but short-circuiting must not commit half a choice.
This extends flag tests into a choice commit. It does not render dialogue or choose the next scene; the caller associates the successful choice with that action. Eight bits suit a small scene; use multiple flag bytes for a larger story.
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
Quest flags: The underlying bits record story state; this routine adds availability and an explicit accepted-choice commit.