Some notes on the *BASIC command

     BBC BASIC does not have a service entry, so cannot check for
the *BASIC command itself. The MOS assists BASIC by providing a
*BASIC command that enters the BASIC ROM. These are some notes
about the *BASIC command and it's associated OSBYTE &BB (187).

     On RESET the MOS sets the BASIC ROM number held in OSBYTE &BB
to &FF. The MOS then builds a table of ROM information scanning for
valid sideways ROMs. It sets the BASIC ROM number to the highest
numbered ROM found with no service entry, indicated by bit 7 of the
ROM type byte being zero.

     The *BASIC command checks the BASIC ROM number. If bit 7 is
clear, then the ROM is entered as a language. If bit 7 is set, then
the *BASIC command is passed to sideways ROMs and the current
filing system as an unknown command.

     Obviously, code can change the BASIC ROM number. In particular
an alternative BASIC ROM could change the BASIC ROM number on
startup. To do this it should set the BASIC ROM number to its own
ROM number if it wishes the *BASIC command to simply enter it as a 
language. It should set the BASIC ROM number to its own ROM number
with bit 7 set to have the *BASIC command passed as an unknown
command. This gives the ROM the opportunity to parse any parameters
after the *BASIC command. Once it has done this, it should then
enter itself as a language with OSBYTE &8E (142). As the BASIC ROM
number is passed to OSBYTE &8E if b7=0 then each bit has the same
interpretation as for OSBYTE &8E.

    The OSBYTE &BB value can be seen as follows:

	    ---------------------------------
	    | E | N |   |   | R | R | R | R |
	    ---------------------------------

	E=0 => Enter ROM number R at language entry point
	E=1 => Pass "BASIC" command to all ROMs service entry point
	N=0 => Automatically relocate if possible
	N=1 => Do not automatically relocate
	R=ROM number of BASIC

     Code similar to the following could be called on service call 1.

	.SERVICE1
	PHA           \ SAVE SERVICE CALL NUMBER
	TYA
	PHA           \ SAVE Y PARAMETER
	TXA           \ HOLDS CURRENT ROM NUMBER
	ORA #&80      \ SET BIT 7
	TAX
	LDA #&BB
	LDY #&00
	JSR OSBYTE    \ WRITE BASIC ROM NUMBER
	LDX &F4       \ RESTORE ROM NUMBER TO X
	PLA
	TAY           \ RESTORE Y PARAMETER
	PLA           \ RESTORE SERVICE CALL NUMBER
	RTS

     Code that reads OSBYTE &BB should check that it equals &FF as
indicating that there is no BASIC present. If it is not &FF, then
bit 7 should be ignored. The following sample code will do this.

	LDA #&BB
	LDX #&00
	LDY #&FF
	JSR OSBYTE    \ READ BASIC ROM NUMBER
	INX
	BEQ NOBASIC   \ ROM NUMBER IS &FF, NO BASIC PRESENT
	DEX           \ X CONTAINS BASIC ROM NUMBER
	TAX
	AND #&7F      \ REMOVE BIT 7

History
-------
  -Nov-1983 Original Acorn document
12-Aug-2006 Added 'N' bit from MOS 3.50 documentation