OSBYTE

OSBYTE performs various functions passing and returning up to two bytes of data.

Specification

6502	Z80	6809	PDP11	80x86	32016	ARM	RISC-V	On entry:	On exit:
A	A	A	R0	AL	R1	R0	A0	= function code	preserved, but see &83 and &84.
X	L	X	R1	BX	R2	R1	A1	= first parameter	returned value
Y	H	Y	R2	CX	R3	R2	A2	= second parameter if A>&7F, ignored and forced to &00 if A<&80	returned value if A>&7F
Cy	Cy	Cy	Cy	Cy	Cy	Cy	Cy		returned value if A>&7F

Calling from BBC BASIC

• Escape is acknowledged with OSBYTE &7E
• =EOF#ch calls OSBYTE &7F
• ADVAL calls OSBYTE &80
• INKEY calls OSBYTE &81
• LOAD, SAVE, CHAIN call OSBYTE &82
• PAGE is initialised with OSBYTE &83
• HIMEM is initialised with OSBYTE &84
• MODE calls OSBYTE &82 then OSBYTE &85
• POS and VPOS call OSBYTE &86
• =MODE calls OSBYTE &87
• The error handler clears the VDU queue with OSBYTE &DA

Entry points

• BBC BASIC Entry Address: &FFF4
• 6502 Entry Address: &FFF4, vectors via &020A
• Z80 Entry Address: &FFF4, vectors via &FFF5
• 6809 Entry Address: &FFF4, vectors via &FFF5
• 80x86 Entry Address: INT &4B, vectors via 0000:012C
• 32000 Entry Address: SVC &06
• PDP11 Entry Address: EMT 2
• ARM Entry Address: SWI &06 "OS_Byte", vector &06
• RISC-V Entry Address: ECALL &AC0002

Implementations

Any OSBYTE call not recognised by the operating system is passed to sideways ROMs. If no sideways ROM recognises the call, then X is returned set to &FF.

The *FX command calls OSBYTE and generates the Bad command error if it is not recognised.

See also

• OSBYTE index

Jgharston 16:58, 6 November 2009 (UTC)