Buffers

MOS maintains nine FIFO buffers servicing slow peripherals — keyboard scan, RS423, printer, sound channels, speech. Producer and consumer run at different rates; the buffer decouples them so the foreground task doesn’t block.

ID	Name	Default size	Producer	Consumer
0	Keyboard input	32 bytes	keyboard IRQ (CA2 on System VIA)	OSRDCH
1	RS423 input	32 bytes	6850 receive IRQ	OSRDCH (when input source = RS423)
2	RS423 output	32 bytes	OSWRCH (when output → RS423)	6850 transmit IRQ
3	Printer	64 bytes	OSWRCH (when printer enabled)	printer driver (User VIA CA1)
4-7	Sound channels 0-3	16 bytes each	OSWORD &07 (SOUND)	100 Hz timer IRQ
8	Speech	—	speech driver	speech chip (Model B/B+ only)

Buffer storage spans &800-&9FF (os-workspace): sound + envelope + (optional) printer in &800-&8FF, CFS/RS423/speech output in &900-&9BF, speech/CFS input in &9C0-&9FF. Per-buffer base addresses, removal pointers, insertion pointers, and busy flags live in OS workspace bytes around &2C3-&2DD on Model B and similar regions on Master.

Vectors

Vector	Address	Purpose
INSV	&22A/&22B	Insert byte
REMV	&22C/&22D	Remove or examine next byte
CNPV	&22E/&22F	Count free / used space, or purge

The default routines validate nothing — passing an out-of-range buffer ID is undefined. Hooks must check X themselves.

Not available across Tube. Buffer hooks must reside on the I/O processor — ideally in a service ROM (paged-roms).

INSV — insert

LDA #value
LDX #buffer_id
JSR (insv_via_macro)  ; or JMP (&22A) from your own handler
; on exit: A, X preserved; Y undefined
; C=1 if buffer was full (insertion failed), C=0 if inserted

REMV — remove or examine

V flag selects mode: V=1 examine, V=0 remove.

LDX #buffer_id
CLV               ; remove (or BIT $XX where XX has bit 6 set, for examine)
JSR remv_chain    ; or JMP (&22C)
; remove: Y = byte just removed, A undefined, X preserved
; examine: A = byte without removing, Y undefined, X preserved
; C=1 if buffer was empty

CNPV — count or purge

V=1 → purge (clear buffer). V=0 → count. C=1 → return free space. C=0 → return content length.

LDX #buffer_id
JSR cnp_chain     ; X+Y = 16-bit count (low+high) for count modes
                  ; X, Y preserved for purge

OSBYTE wrappers (no vector access needed)

OSBYTE	Function	X / Y
&0F (15)	Flush class	X=0 → all buffers; X≠0 → input buffers only
&15 (21)	Flush specific buffer	X = buffer ID (0-8)
&80 (128)	Read buffer status	X = &FF - id (so &FF=kbd, &FC=printer, &FB=sound 0). Returns count in X. Collides with ADC read when X < 4.
&8A (138)	Insert	X = buffer, Y = byte. C=1 if full.
&91 (145)	Get byte	X = buffer. Y = byte. C=1 if empty.
&98 (152)	Examine next	X = buffer. OS 1.20 / Model B: Y = pointer offset from &FA/&FB; later OS: Y = byte itself.
&99 (153)	Insert into input	X = 0 (kbd) or 1 (RS423). Generates event 2 (events). Honours ESCAPE-character (OSBYTE &DC) by generating event 6 if matched.

The OSBYTE &80 ID-inversion is unique to this call. Other buffer OSBYTEs use the direct ID (0-8).

Hooking the vectors

Common reason: replace the printer buffer (default 64 bytes is tiny) with a 4-KB or larger user buffer. Pattern:

1. SEI.
2. Save current &22A-&22F to your handler’s chain locations.
3. Install your INSV/REMV/CNPV addresses.
4. CLI.

Inside each handler:

.my_insv
    CPX #my_buffer_id
    BNE chain_to_os
    ; ... handle insert into your buffer ...
    RTS
.chain_to_os
    JMP (old_insv)

NAUG §9.4 p137-138 has a complete worked example installing a user-defined printer buffer (*FX 21,3 first to flush the old one, then hook). It uses zero-page &80-&83 for the input/output pointer pair — note the conflict with BASIC’s user zp.

Buffer events

The buffer system feeds events (events):

• Event 0 (output buffer empty) — fires just after REMV takes the last byte. X = buffer.
• Event 1 (input buffer full) — fires when INSV would fail. X = buffer, Y = byte that couldn’t fit.
• Event 2 (char entering input buffer) — fires from OSBYTE &99 and the keyboard IRQ. Y = ASCII char.

Performance notes

• Buffer ops are IRQ-driven for hardware-fed buffers (keyboard, 6850, printer ACK, sound timer). Foreground reads/writes via OSWRCH/OSRDCH and OSBYTE wrappers walk the buffers without polling the hardware.
• For maximum throughput to the printer or RS423, bypass MOS entirely: hook the relevant device IRQ yourself, and drive the chip register-by-register. Buffers add per-byte overhead (vector dispatch + ptr math + IRQ wakeup). The trade-off is reimplementing flow control / ACK handshake.
• The sound channel buffers are the one place you can’t really bypass and still use MOS sound — if you want fully custom sound, drive the SN76489 directly (sn76489) and ignore the buffers.

See also

• os-workspace — Buffer RAM at &800-&9BF, pointer table around &2C3-&2DD.
• events — Buffer events 0/1/2.
• system-via — Keyboard CA2, ADC CB1 (consumers).
• user-via — Printer CA1 (printer-buffer consumer).
• sn76489 — Sound chip (sound-buffer consumer).

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This wiki is curated by Claude following the LLM-Wiki methodology — a human curates source documents, the LLM compiles structured cross-linked markdown. Content may contain errors, omissions, or stale claims. For authoritative information refer to the original source documents in the bbc-documents GitHub archive.