Printer Driver

MOS-level printer interface and how to hook a custom printer driver via UPTV (&222).

Selecting the printer

OSBYTE &05 chooses the destination:

X	Destination
0	Sink (output ignored)
1	Parallel port (default on BBC)
2	RS423 (Acorn serial)
3	User printer routine (UPTV)
4	Net printer (via NETV)
5-255	User-defined

*FX 5,X is the user form. The setting survives soft BREAK.

Ignore character

OSBYTE &06 / OSBYTE &F6 set/read a character that’s dropped from print output. Default &0A (LF) — many printers do their own LF after CR, so dropping it prevents double-spacing.

OSBYTE &B6 (Master only) toggles whether the ignore is in effect at all.

UPTV — user print vector (&222)

Hook this to implement a custom printer driver:

LDA &222 : STA old_uptv       ; save the chain
LDA &223 : STA old_uptv+1
LDA #my_uptv MOD 256 : STA &222
LDA #my_uptv DIV 256 : STA &223

MOS calls your hook via UPTV in several scenarios. A = reason code:

A	When	Action
0	Every 10 ms while driver is active	Pull next byte from printer buffer (ID 3) via REMV; send to hardware. When buffer empty, declare dormant via OSBYTE &7B.
1	Buffer became non-empty while dormant	Activate. Return C=0 if you’ve activated; pull a byte and send.
2	VDU 2 received	Printer-on signal (note: characters may still arrive without VDU 2 depending on *FX 3)
3	VDU 3 received	Printer-off signal
5	New printer selected via OSBYTE &5	Update internal state

X = printer buffer ID (always 3). Y = printer number (the *FX 5 value) — only respond if Y matches your printer number.

Standard hook:

.my_uptv
    CPY #my_printer_num    ; my printer ID (e.g. 3, 7, etc.)
    BEQ handle
    JMP (old_uptv)         ; not for us, chain on
 
.handle
    PHA                    ; save reason code
    PHX : PHY
    ; ... process based on A reason code ...
    PLY : PLX
    PLA
    RTS

Declare dormant — OSBYTE &7B

When your buffer is empty, tell MOS to stop polling you:

LDA #&7B : LDX #3 : JSR &FFF4    ; printer driver going dormant

MOS will resume calls (reason A=1) when the buffer gets a byte again.

Pulling bytes — REMV (&22C)

Inside your A=0 / A=1 handler, use buffers REMV to pull bytes from the printer buffer (ID 3):

LDX #3                 ; buffer ID
CLV                    ; V=0 means remove (V=1 = examine only)
JSR remv_chain         ; or JMP (&22C) in chain handlers
; Y = byte; C=1 if buffer was empty

Useful patterns

Serial-attached printer

Already supported via *FX 5,2 (RS423 output). No UPTV hook needed.

Custom hardware printer (e.g. serial DAC for sound)

Use UPTV reason 0 to pull bytes from the printer buffer at 100 Hz and send them as needed. Effectively turns the printer buffer into a 64-byte queue serviced every 10 ms.

Network printer (NETV path)

Already supported via *FX 5,4. NETV reason codes 0/1/2/3/5 mirror UPTV.

Performance notes

• Printer buffer is 64 bytes by default. Filled by OSWRCH when output goes to printer.
• The custom-buffer pattern (NAUG §9.4 worked example, buffers) extends the printer buffer to many KB by intercepting INSV/REMV/CNPV for buffer ID 3.
• For high-throughput output to a fast printer / serial-attached peripheral, use the OSBYTE / UPTV path rather than direct STA to the printer port — the buffered/IRQ-driven path lets the foreground task do other work.

See also

• buffers — Printer buffer (ID 3); INSV/REMV/CNPV vectors.
• user-via — Parallel printer port (Port A + CA1/CA2).
• osbyte — &05, &06, &7B, &F5, &F6 printer entries.
• naug-ch24-misc — Full source page.

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