A partly off-screen object should expose only the part that lies inside the viewport.
Use it in a project
Both rectangles use unsigned byte coordinates and half-open edges [left,right)×[top,bottom). Require ordered edges; equal edges are valid empty rectangles. Return the intersection and VISIBLE=1 if it has area. Empty results are all zero. Invalid reversed edges return VALID=0.
Include component.inc once (MOS callers include the shared 6502 implementation through it). Reserve the public fields in this exact order: OUT_LEFT, OUT_TOP, OUT_RIGHT, OUT_BOTTOM, VISIBLE, VALID, LEFT, TOP, RIGHT, BOTTOM, VIEW_LEFT, VIEW_TOP, VIEW_RIGHT, VIEW_BOTTOM. They occupy 14 contiguous bytes. Also reserve the private workspace: T0, T1, T2, T3 (4 bytes). Array names in the explanation refer to the numbered consecutive bindings, such as C0, C1 and so on.
Call rectangle_clipping_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 rectangle_clipping_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 | both sets of rectangle edges |
| Read after service | intersection edges, VISIBLE and VALID |
The intersection starts at the greater left/top edges and ends at the lesser right/bottom edges. Strictly positive width and height distinguish a visible area from touching edges.
Reusable initialisation and service
; SPDX-License-Identifier: MIT
; Clip a Rectangle to a Viewport. See README.md for the byte interface and ownership.
; init clears the public fields and private workspace; set inputs afterwards.
rectangle_clipping_init:
move.b #0,d0
move.b d0,OUT_LEFT
move.b d0,OUT_TOP
move.b d0,OUT_RIGHT
move.b d0,OUT_BOTTOM
move.b d0,VISIBLE
move.b d0,VALID
move.b d0,LEFT
move.b d0,TOP
move.b d0,RIGHT
move.b d0,BOTTOM
move.b d0,VIEW_LEFT
move.b d0,VIEW_TOP
move.b d0,VIEW_RIGHT
move.b d0,VIEW_BOTTOM
move.b d0,T0
move.b d0,T1
move.b d0,T2
move.b d0,T3
rts
rectangle_clipping_step:
; OUT_LEFT = 0
move.b #0,d0
move.b d0,OUT_LEFT
; OUT_TOP = 0
move.b #0,d0
move.b d0,OUT_TOP
; OUT_RIGHT = 0
move.b #0,d0
move.b d0,OUT_RIGHT
; OUT_BOTTOM = 0
move.b #0,d0
move.b d0,OUT_BOTTOM
; VISIBLE = 0
move.b #0,d0
move.b d0,VISIBLE
; VALID = 0
move.b #0,d0
move.b d0,VALID
; if LEFT > RIGHT:
move.b LEFT,d0
move.b RIGHT,d1
cmp.b d1,d0
bhi rectangle_clipping_L1
jmp rectangle_clipping_L2
rectangle_clipping_L1:
; return
rts
jmp rectangle_clipping_L3
rectangle_clipping_L2:
rectangle_clipping_L3:
; if TOP > BOTTOM:
move.b TOP,d0
move.b BOTTOM,d1
cmp.b d1,d0
bhi rectangle_clipping_L4
jmp rectangle_clipping_L5
rectangle_clipping_L4:
; return
rts
jmp rectangle_clipping_L6
rectangle_clipping_L5:
rectangle_clipping_L6:
; if VIEW_LEFT > VIEW_RIGHT:
move.b VIEW_LEFT,d0
move.b VIEW_RIGHT,d1
cmp.b d1,d0
bhi rectangle_clipping_L7
jmp rectangle_clipping_L8
rectangle_clipping_L7:
; return
rts
jmp rectangle_clipping_L9
rectangle_clipping_L8:
rectangle_clipping_L9:
; if VIEW_TOP > VIEW_BOTTOM:
move.b VIEW_TOP,d0
move.b VIEW_BOTTOM,d1
cmp.b d1,d0
bhi rectangle_clipping_L10
jmp rectangle_clipping_L11
rectangle_clipping_L10:
; return
rts
jmp rectangle_clipping_L12
rectangle_clipping_L11:
rectangle_clipping_L12:
; VALID = 1
move.b #1,d0
move.b d0,VALID
; T0 = LEFT
move.b LEFT,d0
move.b d0,T0
; if VIEW_LEFT > T0:
move.b VIEW_LEFT,d0
move.b T0,d1
cmp.b d1,d0
bhi rectangle_clipping_L13
jmp rectangle_clipping_L14
rectangle_clipping_L13:
; T0 = VIEW_LEFT
move.b VIEW_LEFT,d0
move.b d0,T0
jmp rectangle_clipping_L15
rectangle_clipping_L14:
rectangle_clipping_L15:
; T1 = TOP
move.b TOP,d0
move.b d0,T1
; if VIEW_TOP > T1:
move.b VIEW_TOP,d0
move.b T1,d1
cmp.b d1,d0
bhi rectangle_clipping_L16
jmp rectangle_clipping_L17
rectangle_clipping_L16:
; T1 = VIEW_TOP
move.b VIEW_TOP,d0
move.b d0,T1
jmp rectangle_clipping_L18
rectangle_clipping_L17:
rectangle_clipping_L18:
; T2 = RIGHT
move.b RIGHT,d0
move.b d0,T2
; if VIEW_RIGHT < T2:
move.b VIEW_RIGHT,d0
move.b T2,d1
cmp.b d1,d0
bcs rectangle_clipping_L19
jmp rectangle_clipping_L20
rectangle_clipping_L19:
; T2 = VIEW_RIGHT
move.b VIEW_RIGHT,d0
move.b d0,T2
jmp rectangle_clipping_L21
rectangle_clipping_L20:
rectangle_clipping_L21:
; T3 = BOTTOM
move.b BOTTOM,d0
move.b d0,T3
; if VIEW_BOTTOM < T3:
move.b VIEW_BOTTOM,d0
move.b T3,d1
cmp.b d1,d0
bcs rectangle_clipping_L22
jmp rectangle_clipping_L23
rectangle_clipping_L22:
; T3 = VIEW_BOTTOM
move.b VIEW_BOTTOM,d0
move.b d0,T3
jmp rectangle_clipping_L24
rectangle_clipping_L23:
rectangle_clipping_L24:
; if T0 >= T2:
move.b T0,d0
move.b T2,d1
cmp.b d1,d0
bcc rectangle_clipping_L25
jmp rectangle_clipping_L26
rectangle_clipping_L25:
; return
rts
jmp rectangle_clipping_L27
rectangle_clipping_L26:
rectangle_clipping_L27:
; if T1 >= T3:
move.b T1,d0
move.b T3,d1
cmp.b d1,d0
bcc rectangle_clipping_L28
jmp rectangle_clipping_L29
rectangle_clipping_L28:
; return
rts
jmp rectangle_clipping_L30
rectangle_clipping_L29:
rectangle_clipping_L30:
; OUT_LEFT = T0
move.b T0,d0
move.b d0,OUT_LEFT
; OUT_TOP = T1
move.b T1,d0
move.b d0,OUT_TOP
; OUT_RIGHT = T2
move.b T2,d0
move.b d0,OUT_RIGHT
; OUT_BOTTOM = T3
move.b T3,d0
move.b d0,OUT_BOTTOM
; VISIBLE = 1
move.b #1,d0
move.b d0,VISIBLE
rts
component_end:Machine contract
D0–D3/A0–A1/CCR may change; D4–D7/A2–A6 are preserved. A direct BSR/JSR needs four 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 A500 OCS diagnostic cold-boots an OFS disk, owns the Copper and bitplane and disables interrupt/DMA sources. The helper does not take over hardware or allocate Chip RAM.
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 and BUILD198X=/path/to/build198x. The Makefile produces normal, custom, deliberately wrong and repaired programs. Tools must already be installed; the sample downloads nothing.
Bindings and include order
section code,code_c
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"
even
include "data.inc"
include "cases.inc"
even
include "display.inc"
include "../../framework/turn-deadline/font.inc"
code_end:
STATE:
OUT_LEFT: dc.b 0
OUT_TOP: dc.b 0
OUT_RIGHT: dc.b 0
OUT_BOTTOM: dc.b 0
VISIBLE: dc.b 0
VALID: dc.b 0
LEFT: dc.b 0
TOP: dc.b 0
RIGHT: dc.b 0
BOTTOM: dc.b 0
VIEW_LEFT: dc.b 0
VIEW_TOP: dc.b 0
VIEW_RIGHT: dc.b 0
VIEW_BOTTOM: dc.b 0
T0: dc.b 0
T1: dc.b 0
T2: dc.b 0
T3: dc.b 0
even
ready: dc.w 0
preview: dcb.b 8,0
OUTPUT: dcb.b 1300,0
even
signature: dc.b "198XTOOLKIT0000",0
even
copperlist:
dc.w $08e,$2c81,$090,$2cc1,$092,$0038,$094,$00d0
dc.w $100,$1200,$102,0,$104,0,$108,0,$10a,0
dc.w $180,$012,$182,$fff
copbpl:
dc.w $0e0,0,$0e2,0
dc.w $ffff,$fffe
plane: dcb.b 10240,0A client retaining state between calls
preview_rules:
ifne CUSTOM
; LEFT = 0
move.b #0,d0
move.b d0,LEFT
; TOP = 0
move.b #0,d0
move.b d0,TOP
; RIGHT = 255
move.b #255,d0
move.b d0,RIGHT
; BOTTOM = 255
move.b #255,d0
move.b d0,BOTTOM
; VIEW_LEFT = 10
move.b #10,d0
move.b d0,VIEW_LEFT
; VIEW_TOP = 20
move.b #20,d0
move.b d0,VIEW_TOP
; VIEW_RIGHT = 30
move.b #30,d0
move.b d0,VIEW_RIGHT
; VIEW_BOTTOM = 40
move.b #40,d0
move.b d0,VIEW_BOTTOM
ifne BROKEN
; VIEW_RIGHT = 255
move.b #255,d0
move.b d0,VIEW_RIGHT
endif
bsr checked_step
bsr emit_state
rts
else
; LEFT = 2
move.b #2,d0
move.b d0,LEFT
; TOP = 3
move.b #3,d0
move.b d0,TOP
; RIGHT = 12
move.b #12,d0
move.b d0,RIGHT
; BOTTOM = 15
move.b #15,d0
move.b d0,BOTTOM
; VIEW_LEFT = 5
move.b #5,d0
move.b d0,VIEW_LEFT
; VIEW_TOP = 0
move.b #0,d0
move.b d0,VIEW_TOP
; VIEW_RIGHT = 10
move.b #10,d0
move.b d0,VIEW_RIGHT
; VIEW_BOTTOM = 10
move.b #10,d0
move.b d0,VIEW_BOTTOM
ifne BROKEN
; VIEW_RIGHT = 255
move.b #255,d0
move.b d0,VIEW_RIGHT
endif
bsr checked_step
bsr emit_state
endif
rtsCustomise it
Clip a maximum-size rectangle into a small inset viewport. 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
section code,code_c
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"
even
include "data.inc"
include "cases.inc"
even
include "display.inc"
include "../../framework/turn-deadline/font.inc"
code_end:
STATE:
OUT_LEFT: dc.b 0
OUT_TOP: dc.b 0
OUT_RIGHT: dc.b 0
OUT_BOTTOM: dc.b 0
VISIBLE: dc.b 0
VALID: dc.b 0
LEFT: dc.b 0
TOP: dc.b 0
RIGHT: dc.b 0
BOTTOM: dc.b 0
VIEW_LEFT: dc.b 0
VIEW_TOP: dc.b 0
VIEW_RIGHT: dc.b 0
VIEW_BOTTOM: dc.b 0
T0: dc.b 0
T1: dc.b 0
T2: dc.b 0
T3: dc.b 0
even
ready: dc.w 0
preview: dcb.b 8,0
OUTPUT: dcb.b 1300,0
even
signature: dc.b "198XTOOLKIT0000",0
even
copperlist:
dc.w $08e,$2c81,$090,$2cc1,$092,$0038,$094,$00d0
dc.w $100,$1200,$102,0,$104,0,$108,0,$10a,0
dc.w $180,$012,$182,$fff
copbpl:
dc.w $0e0,0,$0e2,0
dc.w $ffff,$fffe
plane: dcb.b 10240,0The custom diagnostic row is 0A 14 1E 28 01 01 00 00. The full client trace in the source explains how it gets there.
Mistake and repair
The wrong caller replaces the viewport’s right edge with the coordinate ceiling. The clipped object leaks past the intended panel. Supply the actual viewport bounds.
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
section code,code_c
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"
even
include "data.inc"
include "cases.inc"
even
include "display.inc"
include "../../framework/turn-deadline/font.inc"
code_end:
STATE:
OUT_LEFT: dc.b 0
OUT_TOP: dc.b 0
OUT_RIGHT: dc.b 0
OUT_BOTTOM: dc.b 0
VISIBLE: dc.b 0
VALID: dc.b 0
LEFT: dc.b 0
TOP: dc.b 0
RIGHT: dc.b 0
BOTTOM: dc.b 0
VIEW_LEFT: dc.b 0
VIEW_TOP: dc.b 0
VIEW_RIGHT: dc.b 0
VIEW_BOTTOM: dc.b 0
T0: dc.b 0
T1: dc.b 0
T2: dc.b 0
T3: dc.b 0
even
ready: dc.w 0
preview: dcb.b 8,0
OUTPUT: dcb.b 1300,0
even
signature: dc.b "198XTOOLKIT0000",0
even
copperlist:
dc.w $08e,$2c81,$090,$2cc1,$092,$0038,$094,$00d0
dc.w $100,$1200,$102,0,$104,0,$108,0,$10a,0
dc.w $180,$012,$182,$fff
copbpl:
dc.w $0e0,0,$0e2,0
dc.w $ffff,$fffe
plane: dcb.b 10240,0
Cost and alternatives
The measured helper extent is 528 code bytes; public state is 14 bytes and private scratch is 4 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. Clip before entering expensive pixel loops; complex shapes still require their own clipping rules.
This returns geometry, not a masked draw. Coordinates cannot represent an exclusive edge of 256; use a wider type for a full 256-pixel span.
Execution evidence
Four variants run on A500 OCS PAL, Kickstart1.3 OFS cold boot, using Asm198x 0.0.72, Emu198x 0.44.2 and Build198x 0.2.9. 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
Rectangle overlap: Half-open edge conventions must agree between collision and drawing.