OSBYTE

From BeebWiki

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

Contents 1 Specification 2 Calling from BBC BASIC 3 Entry points 4 Implementations

Specification

On entry:
A	= function code
X	= first parameter
Y	= second parameter if A>=&80, ignored and forced to be &00 if A<&80

On exit:
A	= preserved
X	If A<&80: X = returned value, X=&FF if unrecognised, Y, Cy undefined
XY	If A>=&80: XY = returned value, X=&FF if unrecognised, Cy = returned Carry flag

Calling from BBC BASIC

• Escape is acknowledged with OSBYTE 126
• =EOF#ch calls OSBYTE 127,ch
• ADVAL calls OSBYTE 128
• INKEY calls OSBYTE 129
• LOAD, SAVE, CHAIN call OSBYTE 130
• PAGE is initialised with OSBYTE 131
• HIMEM is initialised with OSBYTE 132
• MODE calls OSBYTE 130 then OSBYTE 133
• POS and VPOS call OSBYTE 134
• =MODE calls OSBYTE 135

Entry points

• BBC BASIC Entry Address: &FFF4
• 6502 Entry Address: &FFF4, vectors via &020A
• Z80 Entry Address: &FFF4, vectors via &FFF5
• 6809 Entry Address:
• 80x86 Entry Address: INT &4B, vectors via 0000:012C
• 32000 Entry Address: SVC &06
• PDP-11 Entry Address: EMT 2
• ARM Entry Address: SWI &06 "OS_Byte", vector &06

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.

Jgharston 06:21, 8 October 2007 (BST)