Code Like It's 198x

Keep a Menu Selection Visible on Spectrum

A list can hold more choices than the screen has rows while its cursor still stays in view.

A list can hold more choices than the screen has rows while its cursor still stays in view.

Spectrum shows the working diagnostic row 03 00 04 01 03 14 04 00.
A completed native caller displays 03 00 04 01 03 14 04 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

ITEM_COUNT and PAGE_ROWS must be nonzero and SELECTED below ITEM_COUNT. TOP is a retained requested first row; clamp it to max(0, ITEM_COUNT-PAGE_ROWS). Shift it just enough to contain SELECTED. Return ROW=SELECTED-TOP and VISIBLE=min(PAGE_ROWS, ITEM_COUNT-TOP). Invalid input clears ROW/VISIBLE/VALID and preserves TOP. Selection changes, disabled choices and input repeat remain caller-owned.

Include component.inc once (MOS callers include the shared 6502 implementation through it). Reserve the public fields in this exact order: TOP, ROW, VISIBLE, VALID, SELECTED, ITEM_COUNT, PAGE_ROWS. They occupy 7 contiguous bytes. Also reserve the private workspace: T0, T1 (2 bytes). Array names in the explanation refer to the numbered consecutive bindings, such as C0, C1 and so on.

Call menu_page_window_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 menu_page_window_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 TOP
Supply before service SELECTED, ITEM_COUNT and PAGE_ROWS
Read after service TOP, ROW, VISIBLE and VALID

Clamp the old window against the list, then move only when selection crosses either edge. Compute relative differences instead of adding page size to TOP, avoiding byte wrap near the end of a long list.

Reusable initialisation and service

component.inc
; SPDX-License-Identifier: MIT
; Keep a Menu Selection Visible. See README.md for the byte interface and ownership.
; init clears the public fields and private workspace; set inputs afterwards.
menu_page_window_init:
    ld a,0
    ld (TOP),a
    ld (ROW),a
    ld (VISIBLE),a
    ld (VALID),a
    ld (SELECTED),a
    ld (ITEM_COUNT),a
    ld (PAGE_ROWS),a
    ld (T0),a
    ld (T1),a
    ret
menu_page_window_step:
; ROW = 0
    ld a,0
    ld (ROW),a
; VISIBLE = 0
    ld a,0
    ld (VISIBLE),a
; VALID = 0
    ld a,0
    ld (VALID),a
; if ITEM_COUNT == 0:
    ld a,(ITEM_COUNT)
    cp 0
    jp z,menu_page_window_L1
    jp menu_page_window_L2
menu_page_window_L1:
; return
    ret
    jp menu_page_window_L3
menu_page_window_L2:
menu_page_window_L3:
; if PAGE_ROWS == 0:
    ld a,(PAGE_ROWS)
    cp 0
    jp z,menu_page_window_L4
    jp menu_page_window_L5
menu_page_window_L4:
; return
    ret
    jp menu_page_window_L6
menu_page_window_L5:
menu_page_window_L6:
; if SELECTED >= ITEM_COUNT:
    ld a,(SELECTED)
    ld b,a
    ld a,(ITEM_COUNT)
    ld c,a
    ld a,b
    cp c
    jp nc,menu_page_window_L7
    jp menu_page_window_L8
menu_page_window_L7:
; return
    ret
    jp menu_page_window_L9
menu_page_window_L8:
menu_page_window_L9:
; VALID = 1
    ld a,1
    ld (VALID),a
; T0 = 0
    ld a,0
    ld (T0),a
; if ITEM_COUNT > PAGE_ROWS:
    ld a,(ITEM_COUNT)
    ld b,a
    ld a,(PAGE_ROWS)
    ld c,a
    ld a,b
    cp c
    jp z,menu_page_window_L11
    jp nc,menu_page_window_L10
    jp menu_page_window_L11
menu_page_window_L10:
; T0 = ITEM_COUNT - PAGE_ROWS
    ld a,(ITEM_COUNT)
    ld b,a
    ld a,(PAGE_ROWS)
    ld c,a
    ld a,b
    sub c
    ld (T0),a
    jp menu_page_window_L12
menu_page_window_L11:
menu_page_window_L12:
; if TOP > T0:
    ld a,(TOP)
    ld b,a
    ld a,(T0)
    ld c,a
    ld a,b
    cp c
    jp z,menu_page_window_L14
    jp nc,menu_page_window_L13
    jp menu_page_window_L14
menu_page_window_L13:
; TOP = T0
    ld a,(T0)
    ld (TOP),a
    jp menu_page_window_L15
menu_page_window_L14:
menu_page_window_L15:
; if SELECTED < TOP:
    ld a,(SELECTED)
    ld b,a
    ld a,(TOP)
    ld c,a
    ld a,b
    cp c
    jp c,menu_page_window_L16
    jp menu_page_window_L17
menu_page_window_L16:
; TOP = SELECTED
    ld a,(SELECTED)
    ld (TOP),a
    jp menu_page_window_L18
menu_page_window_L17:
; T1 = SELECTED - TOP
    ld a,(SELECTED)
    ld b,a
    ld a,(TOP)
    ld c,a
    ld a,b
    sub c
    ld (T1),a
; if T1 >= PAGE_ROWS:
    ld a,(T1)
    ld b,a
    ld a,(PAGE_ROWS)
    ld c,a
    ld a,b
    cp c
    jp nc,menu_page_window_L19
    jp menu_page_window_L20
menu_page_window_L19:
; T1 = SELECTED - PAGE_ROWS
    ld a,(SELECTED)
    ld b,a
    ld a,(PAGE_ROWS)
    ld c,a
    ld a,b
    sub c
    ld (T1),a
; TOP = T1 + 1
    ld a,(T1)
    add a,1
    ld (TOP),a
    jp menu_page_window_L21
menu_page_window_L20:
menu_page_window_L21:
menu_page_window_L18:
; ROW = SELECTED - TOP
    ld a,(SELECTED)
    ld b,a
    ld a,(TOP)
    ld c,a
    ld a,b
    sub c
    ld (ROW),a
; VISIBLE = ITEM_COUNT - TOP
    ld a,(ITEM_COUNT)
    ld b,a
    ld a,(TOP)
    ld c,a
    ld a,b
    sub c
    ld (VISIBLE),a
; if VISIBLE > PAGE_ROWS:
    ld a,(VISIBLE)
    ld b,a
    ld a,(PAGE_ROWS)
    ld c,a
    ld a,b
    cp c
    jp z,menu_page_window_L23
    jp nc,menu_page_window_L22
    jp menu_page_window_L23
menu_page_window_L22:
; VISIBLE = PAGE_ROWS
    ld a,(PAGE_ROWS)
    ld (VISIBLE),a
    jp menu_page_window_L24
menu_page_window_L23:
menu_page_window_L24:
    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

demo.asm
    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:
TOP: defb 0
ROW: defb 0
VISIBLE: defb 0
VALID: defb 0
SELECTED: defb 0
ITEM_COUNT: defb 0
PAGE_ROWS: defb 0
T0: defb 0
T1: 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

exercise.inc, excerpt
preview_rules:
    if CUSTOM
; ITEM_COUNT = 255
    ld a,255
    ld (ITEM_COUNT),a
; PAGE_ROWS = 8
    ld a,8
    ld (PAGE_ROWS),a
; SELECTED = 254
    ld a,254
    ld (SELECTED),a
; TOP = 255
    ld a,255
    ld (TOP),a
    if BROKEN
; TOP = 0
    ld a,0
    ld (TOP),a
    endif
    call checked_step
    call emit_state
    ret
    else
; ITEM_COUNT = 20
    ld a,20
    ld (ITEM_COUNT),a
; PAGE_ROWS = 4
    ld a,4
    ld (PAGE_ROWS),a
; SELECTED = 5
    ld a,5
    ld (SELECTED),a
    if BROKEN
; TOP = 0
    ld a,0
    ld (TOP),a
    endif
    call checked_step
    call emit_state
; SELECTED = 6
    ld a,6
    ld (SELECTED),a
    if BROKEN
; TOP = 0
    ld a,0
    ld (TOP),a
    endif
    call checked_step
    call emit_state
; SELECTED = 3
    ld a,3
    ld (SELECTED),a
    if BROKEN
; TOP = 0
    ld a,0
    ld (TOP),a
    endif
    call checked_step
    call emit_state
    endif
    ret

Customise it

Show the last item of a 255-entry list in an eight-row page. 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
    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:
TOP: defb 0
ROW: defb 0
VISIBLE: defb 0
VALID: defb 0
SELECTED: defb 0
ITEM_COUNT: defb 0
PAGE_ROWS: defb 0
T0: defb 0
T1: 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 F7 07 08 01 FE FF 08 00. The full client trace in the source explains how it gets there.

Mistake and repair

The wrong client resets the retained top row before each call. Moving back to row three jumps the list to the beginning instead of preserving the nearby window. Retain TOP between selection changes.

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
    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:
TOP: defb 0
ROW: defb 0
VISIBLE: defb 0
VALID: defb 0
SELECTED: defb 0
ITEM_COUNT: defb 0
PAGE_ROWS: defb 0
T0: defb 0
T1: 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
Spectrum shows the deliberate error row 00 03 04 01 03 14 04 00.
The wrong client completes with 00 03 04 01 03 14 04 00; its changed inputs and state explain the result.

Cost and alternatives

The measured helper extent is 301 code bytes; public state is 7 bytes and private scratch is 2 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 regardless of list length. Redraw only changed rows when your display path permits it; the helper does not promise that scrolling a bitmap costs the same as changing a hardware character map.

The helper returns logical indices, not character or pixel positions. Count255 means valid indices0..254. An empty list needs a separate empty-state view. For variable-height rows, use a pixel-offset viewport rather than this fixed-row policy.

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

Menu navigation: Resolve selection and disabled options first, then use this window to choose which rows to draw.

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