Code Like It's 198x

Gate a Dialogue Choice with Flags on C64

A dialogue option can depend on what the player knows and change what happens next.

A dialogue option can depend on what the player knows and change what happens next.

C64 shows the working diagnostic row 02 01 02 02 01 01 00 00.
A completed native caller displays 02 01 02 02 01 01 00 00. The public field order below identifies these bytes; the capture is a state diagnostic, not a recording of a finished game.

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

6502.inc
; 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:
    lda #0
    sta FLAGS
    sta REQUIRED
    sta FORBIDDEN
    sta SET_MASK
    sta CLEAR_MASK
    sta ACCEPT
    sta AVAILABLE
    sta TAKEN
    sta VALID
    sta T0
    rts
dialogue_gates_step:
; AVAILABLE = 0
    lda #0
    sta AVAILABLE
; TAKEN = 0
    lda #0
    sta TAKEN
; VALID = 0
    lda #0
    sta VALID
; T0 = REQUIRED & FORBIDDEN
    lda REQUIRED
    and FORBIDDEN
    sta T0
; if T0 != 0:
    lda T0
    cmp #0
    bne dialogue_gates_L4
dialogue_gates_L5:
    jmp dialogue_gates_L2
dialogue_gates_L4:
    jmp dialogue_gates_L1
dialogue_gates_L1:
; return
    rts
    jmp dialogue_gates_L3
dialogue_gates_L2:
dialogue_gates_L3:
; VALID = 1
    lda #1
    sta VALID
; T0 = FLAGS & REQUIRED
    lda FLAGS
    and REQUIRED
    sta T0
; if T0 != REQUIRED:
    lda T0
    cmp REQUIRED
    bne dialogue_gates_L9
dialogue_gates_L10:
    jmp dialogue_gates_L7
dialogue_gates_L9:
    jmp dialogue_gates_L6
dialogue_gates_L6:
; return
    rts
    jmp dialogue_gates_L8
dialogue_gates_L7:
dialogue_gates_L8:
; T0 = FLAGS & FORBIDDEN
    lda FLAGS
    and FORBIDDEN
    sta T0
; if T0 != 0:
    lda T0
    cmp #0
    bne dialogue_gates_L14
dialogue_gates_L15:
    jmp dialogue_gates_L12
dialogue_gates_L14:
    jmp dialogue_gates_L11
dialogue_gates_L11:
; return
    rts
    jmp dialogue_gates_L13
dialogue_gates_L12:
dialogue_gates_L13:
; AVAILABLE = 1
    lda #1
    sta AVAILABLE
; if ACCEPT == 0:
    lda ACCEPT
    cmp #0
    beq dialogue_gates_L19
dialogue_gates_L20:
    jmp dialogue_gates_L17
dialogue_gates_L19:
    jmp dialogue_gates_L16
dialogue_gates_L16:
; return
    rts
    jmp dialogue_gates_L18
dialogue_gates_L17:
dialogue_gates_L18:
; T0 = CLEAR_MASK ^ 255
    lda CLEAR_MASK
    eor #255
    sta T0
; FLAGS = FLAGS & T0
    lda FLAGS
    and T0
    sta FLAGS
; FLAGS = FLAGS | SET_MASK
    lda FLAGS
    ora SET_MASK
    sta FLAGS
; TAKEN = 1
    lda #1
    sta TAKEN
    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. Scope or rename routine and field labels and allocate a separate block for each independent instance. 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

demo.asm
FLAGS=$80
REQUIRED=$81
FORBIDDEN=$82
SET_MASK=$83
CLEAR_MASK=$84
ACCEPT=$85
AVAILABLE=$86
TAKEN=$87
VALID=$88
T0=$89
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

exercise.inc, excerpt
preview_rules:
!if CUSTOM {
; FLAGS = 5
    lda #5
    sta FLAGS
; REQUIRED = 5
    lda #5
    sta REQUIRED
; FORBIDDEN = 2
    lda #2
    sta FORBIDDEN
; SET_MASK = 2
    lda #2
    sta SET_MASK
; CLEAR_MASK = 1
    lda #1
    sta CLEAR_MASK
; ACCEPT = 0
    lda #0
    sta ACCEPT
!if BROKEN {
; FLAGS = 0
    lda #0
    sta FLAGS
}
    jsr checked_step
    jsr emit_state
; ACCEPT = 1
    lda #1
    sta ACCEPT
!if BROKEN {
; FLAGS = 0
    lda #0
    sta FLAGS
}
    jsr checked_step
    jsr emit_state

!if BROKEN {
; FLAGS = 0
    lda #0
    sta FLAGS
}
    jsr checked_step
    jsr emit_state
    rts
} else {
; FLAGS = 1
    lda #1
    sta FLAGS
; REQUIRED = 1
    lda #1
    sta REQUIRED
; FORBIDDEN = 2
    lda #2
    sta FORBIDDEN
; SET_MASK = 2
    lda #2
    sta SET_MASK
; CLEAR_MASK = 1
    lda #1
    sta CLEAR_MASK
; ACCEPT = 0
    lda #0
    sta ACCEPT
!if BROKEN {
; FLAGS = 0
    lda #0
    sta FLAGS
}
    jsr checked_step
    jsr emit_state
; ACCEPT = 1
    lda #1
    sta ACCEPT
!if BROKEN {
; FLAGS = 0
    lda #0
    sta FLAGS
}
    jsr checked_step
    jsr emit_state

!if BROKEN {
; FLAGS = 0
    lda #0
    sta FLAGS
}
    jsr checked_step
    jsr emit_state
}
    rts

Customise 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

custom.asm
FLAGS=$80
REQUIRED=$81
FORBIDDEN=$82
SET_MASK=$83
CLEAR_MASK=$84
ACCEPT=$85
AVAILABLE=$86
TAKEN=$87
VALID=$88
T0=$89
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 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

wrong.asm
FLAGS=$80
REQUIRED=$81
FORBIDDEN=$82
SET_MASK=$83
CLEAR_MASK=$84
ACCEPT=$85
AVAILABLE=$86
TAKEN=$87
VALID=$88
T0=$89
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:
C64 shows the deliberate error row 00 01 02 02 01 01 00 00.
The wrong client completes with 00 01 02 02 01 01 00 00; its changed inputs and state explain the result.

Cost and alternatives

The measured helper extent is 148 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 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

Quest flags: The underlying bits record story state; this routine adds availability and an explicit accepted-choice commit.

The Craft · Maths for Games · Source and native evidence.