Count Active Play Time on Spectrum

A pause screen should not quietly spend the player’s time allowance.

Use it in a project

RESET takes priority and clears the 16-bit low-byte-first active counter. Otherwise PAUSED nonzero freezes it; a running call adds one modulo 65,536. Inputs remain caller-owned. The clock measures serviced active ticks, not seconds.

Include component.inc once (MOS callers include the shared 6502 implementation through it). Reserve the public fields in this exact order: TIME_LO, TIME_HI, PAUSED, RESET. They occupy 4 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 pause_clock_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 pause_clock_step whenever the described event occurs. The helper reads no device, waits for no frame and installs no interrupt handler.

A display frame and an active play tick are different events. The clock deliberately counts only the latter. You can keep an always-running clock alongside it for menus or pause-screen animation.

Reusable initialisation and service

; SPDX-License-Identifier: MIT
; Count Active Play Time. See README.md for the byte interface and ownership.
; init clears the public fields and private workspace; set inputs afterwards.
pause_clock_init:
    ld a,0
    ld (TIME_LO),a
    ld (TIME_HI),a
    ld (PAUSED),a
    ld (RESET),a
    ret
pause_clock_step:
; if RESET != 0:
    ld a,(RESET)
    cp 0
    jp nz,pause_clock_L1
    jp pause_clock_L2
pause_clock_L1:
; TIME_LO = 0
    ld a,0
    ld (TIME_LO),a
; TIME_HI = 0
    ld a,0
    ld (TIME_HI),a
; return
    ret
    jp pause_clock_L3
pause_clock_L2:
pause_clock_L3:
; if PAUSED != 0:
    ld a,(PAUSED)
    cp 0
    jp nz,pause_clock_L4
    jp pause_clock_L5
pause_clock_L4:
; return
    ret
    jp pause_clock_L6
pause_clock_L5:
pause_clock_L6:
; TIME_LO = TIME_LO + 1
    ld a,(TIME_LO)
    add a,1
    ld (TIME_LO),a
; if TIME_LO == 0:
    ld a,(TIME_LO)
    cp 0
    jp z,pause_clock_L7
    jp pause_clock_L8
pause_clock_L7:
; TIME_HI = TIME_HI + 1
    ld a,(TIME_HI)
    add a,1
    ld (TIME_HI),a
    jp pause_clock_L9
pause_clock_L8:
pause_clock_L9:
    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:
TIME_LO: defb 0
TIME_HI: defb 0
PAUSED: defb 0
RESET: 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

A client retaining state between calls

preview_rules:
    if CUSTOM
; TIME_LO = 255
    ld a,255
    ld (TIME_LO),a
    if BROKEN
; PAUSED = 0
    ld a,0
    ld (PAUSED),a
    endif
    call checked_step
    call emit_state

    if BROKEN
; PAUSED = 0
    ld a,0
    ld (PAUSED),a
    endif
    call checked_step
    call emit_state
; PAUSED = 1
    ld a,1
    ld (PAUSED),a
    if BROKEN
; PAUSED = 0
    ld a,0
    ld (PAUSED),a
    endif
    call checked_step
    call emit_state

    if BROKEN
; PAUSED = 0
    ld a,0
    ld (PAUSED),a
    endif
    call checked_step
    call emit_state
; PAUSED = 0
    ld a,0
    ld (PAUSED),a
    if BROKEN
; PAUSED = 0
    ld a,0
    ld (PAUSED),a
    endif
    call checked_step
    call emit_state

    if BROKEN
; PAUSED = 0
    ld a,0
    ld (PAUSED),a
    endif
    call checked_step
    call emit_state
    ret
    else

    if BROKEN
; PAUSED = 0
    ld a,0
    ld (PAUSED),a
    endif
    call checked_step
    call emit_state

    if BROKEN
; PAUSED = 0
    ld a,0
    ld (PAUSED),a
    endif
    call checked_step
    call emit_state
; PAUSED = 1
    ld a,1
    ld (PAUSED),a
    if BROKEN
; PAUSED = 0
    ld a,0
    ld (PAUSED),a
    endif
    call checked_step
    call emit_state

    if BROKEN
; PAUSED = 0
    ld a,0
    ld (PAUSED),a
    endif
    call checked_step
    call emit_state
; PAUSED = 0
    ld a,0
    ld (PAUSED),a
    if BROKEN
; PAUSED = 0
    ld a,0
    ld (PAUSED),a
    endif
    call checked_step
    call emit_state

    if BROKEN
; PAUSED = 0
    ld a,0
    ld (PAUSED),a
    endif
    call checked_step
    call emit_state
    endif
    ret

Customise it

Begin at 255 to exercise the carry into the high byte. 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:
TIME_LO: defb 0
TIME_HI: defb 0
PAUSED: defb 0
RESET: 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

The custom diagnostic row is 03 01 00 00 00 00 00 00. The full client trace in the source explains how it gets there.

Mistake and repair

The wrong caller forces PAUSED to zero. Calls made while the pause screen is open still spend active play time. Feed the actual pause state, or deliberately stop servicing the active clock.

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:
TIME_LO: defb 0
TIME_HI: defb 0
PAUSED: defb 0
RESET: 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 86 code bytes; public state is 4 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; carry into the high byte occurs only on a low-byte wrap. A wider counter extends the range at the cost of state and carry handling.

Do not mix this active clock with a deadline taken from a continuously running hardware counter. Longer sessions need wider storage or explicit wrap-aware comparisons.

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

Rational tick division: Rate division decides when a tick is due; the pause policy decides whether active play should consume it.

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