Code Like It's 198x

Mirror an Eight-bit Bitmap Row on NES

A stored row of pixels can face the other way without keeping a second copy of the artwork.

A stored row of pixels can face the other way without keeping a second copy of the artwork.

NES shows the working diagnostic row 03 C0 00 00 00 00 00 00.
A completed native caller displays 03 C0 00 00 00 00 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

Reverse all eight bits of ROW into MIRRORED, preserving ROW. Bit seven is the leftmost source pixel. Apply the helper separately to each row of an eight-pixel-wide monochrome image.

Include component.inc once (MOS callers include the shared 6502 implementation through it). Reserve the public fields in this exact order: MIRRORED, ROW. They occupy 2 contiguous bytes. Also reserve the private workspace: T0, T1, T2 (3 bytes). Array names in the explanation refer to the numbered consecutive bindings, such as C0, C1 and so on.

Call bitmap_mirror_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 bitmap_mirror_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 none
Supply before service ROW
Read after service MIRRORED

Consume source bits from the low end while shifting the result left. After eight steps the bit order has reversed exactly.

Reusable initialisation and service

6502.inc
; SPDX-License-Identifier: MIT
; Mirror an Eight-bit Bitmap Row. See README.md for the byte interface and ownership.
; init clears the public fields and private workspace; set inputs afterwards.
bitmap_mirror_init:
    lda #0
    sta MIRRORED
    sta ROW
    sta T0
    sta T1
    sta T2
    rts
bitmap_mirror_step:
; MIRRORED = 0
    lda #0
    sta MIRRORED
; T0 = ROW
    lda ROW
    sta T0
; T1 = 8
    lda #8
    sta T1
; while T1 != 0:
bitmap_mirror_L1:
    lda T1
    cmp #0
    bne bitmap_mirror_L4
bitmap_mirror_L5:
    jmp bitmap_mirror_L3
bitmap_mirror_L4:
    jmp bitmap_mirror_L2
bitmap_mirror_L2:
; T2 = T0 & 1
    lda T0
    and #1
    sta T2
; MIRRORED = MIRRORED << 1
    lda MIRRORED
    asl
    sta MIRRORED
; MIRRORED = MIRRORED | T2
    lda MIRRORED
    ora T2
    sta MIRRORED
; T0 = T0 >> 1
    lda T0
    lsr
    sta T0
; T1 = T1 - 1
    lda T1
    sec
    sbc #1
    sta T1
    jmp bitmap_mirror_L1
bitmap_mirror_L3:
    rts
component_end:

Machine contract

A/X/status may change; Y is preserved. The 2A03 has no decimal arithmetic mode. 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. 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 NROM caller owns PPU setup and background display, with NMI and DMC disabled. Table and packet sizes are bounded to fit the cartridge caller and internal RAM; the helper owns no PPU registers.

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
.segment "HEADER"
.byte "NES",$1a,2,1,0,0,0,0,0,0,0,0,0,0
MIRRORED=$80
ROW=$81
T0=$82
T1=$83
T2=$84
STATE=$80
input_ptr=$70
cases_left=$72
ptr=$74
saved_y=$76
OUTPUT=$0200
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
.include "caller.inc"
.include "exercise.inc"
component_start:
.include "component.inc"
.include "data.inc"
.include "cases.inc"
code_end:
.segment "VECTORS"
.word nmi,reset,irq
.segment "CHARS"
.include "../../input/controller-reading/edges-font.inc"
.res 8192-272,0

A client retaining state between calls

exercise.inc, excerpt
preview_rules:
.if CUSTOM
; ROW = 129
    lda #129
    sta ROW
.if BROKEN
; ROW = ROW & 15
    lda ROW
    and #15
    sta ROW
.endif
    jsr checked_step
    jsr emit_state
    rts
.else
; ROW = 192
    lda #192
    sta ROW
.if BROKEN
; ROW = ROW & 15
    lda ROW
    and #15
    sta ROW
.endif
    jsr checked_step
    jsr emit_state
.endif
    rts

Customise it

Mirror a symmetric row to show that some images legitimately remain unchanged. 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
.segment "HEADER"
.byte "NES",$1a,2,1,0,0,0,0,0,0,0,0,0,0
MIRRORED=$80
ROW=$81
T0=$82
T1=$83
T2=$84
STATE=$80
input_ptr=$70
cases_left=$72
ptr=$74
saved_y=$76
OUTPUT=$0200
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
.include "caller.inc"
.include "exercise.inc"
component_start:
.include "component.inc"
.include "data.inc"
.include "cases.inc"
code_end:
.segment "VECTORS"
.word nmi,reset,irq
.segment "CHARS"
.include "../../input/controller-reading/edges-font.inc"
.res 8192-272,0

The custom diagnostic row is 81 81 00 00 00 00 00 00. The full client trace in the source explains how it gets there.

Mistake and repair

The wrong caller removes the upper nibble before reversal. Pixels at the left vanish rather than moving to the right. Preserve the complete row.

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
.segment "HEADER"
.byte "NES",$1a,2,1,0,0,0,0,0,0,0,0,0,0
MIRRORED=$80
ROW=$81
T0=$82
T1=$83
T2=$84
STATE=$80
input_ptr=$70
cases_left=$72
ptr=$74
saved_y=$76
OUTPUT=$0200
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
.include "caller.inc"
.include "exercise.inc"
component_start:
.include "component.inc"
.include "data.inc"
.include "cases.inc"
code_end:
.segment "VECTORS"
.word nmi,reset,irq
.segment "CHARS"
.include "../../input/controller-reading/edges-font.inc"
.res 8192-272,0
NES shows the deliberate error row 00 00 00 00 00 00 00 00.
The wrong client completes with 00 00 00 00 00 00 00 00; its changed inputs and state explain the result.

Cost and alternatives

The measured helper extent is 70 code bytes; public state is 2 bytes and private scratch is 3 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.

Eight stages per row. A 256-byte reversal table saves runtime; NES sprite horizontal flip avoids transforming pattern bytes altogether. The C64 has no VIC-II sprite flip bit.

This transforms monochrome row data only. It does not change screen attributes, planar colour ordering or sprite registers. NES sprite flip attributes can avoid this work. The C64 needs transformed bitmap data because its sprites have no horizontal-flip bit.

Execution evidence

Four variants run on NTSC NROM, background only, DMC/NMI disabled, 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.

Existing alternatives

The existing full-glyph transform supports quarter-turns, reflections and overlapping source/destination buffers on its documented machines. Its pointer interface and snapshot workspace are useful for whole-font tools. This row helper needs no source/destination pointer or eight-byte snapshot; call it for one row at a time, or choose the full transform for its richer contract.

Where it fits

Custom glyphs: Transform the bitmap representation before translating it into each display’s character or sprite layout.

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