The BBC and Master Computer Public Domain Library

8000 BASIC ROM header

Submitted by Steve Fewell

Description:

The first 6 bytes contain code to jump to the BASIC ROM startup routine.
This code does the following:
      If Accumulator is 1 then jump to start of the language address (&802B);
      otherwise, exit (RTS) ('A=1' indicates a normal language entry).
      The NOP command in byte &8005 is just to fill space!

Any service address 'jump to startup' code is usually in bytes &8003 to &8005, but
as BASIC doesn't have a service address, this space is used to extend the jump to
the startup routine.

Byte &8006 = ROM Type. &60 indicates a language ROM (i.e. it has a language entry address
& a second processor relocation address).
ROM Type &60 [0110 0000] indicates that:
     * No Service entry point is present (Bit 7 is clear)
     * A Language entry point is present (Bit 6 is set)
     * A 2nd processor relocation address is present (Bit 5 is set)
     * ROM does not support Electron firmkeys (Bit 4 is clear)
Bits 0-3 provide an ID for the ROM - 0000 indicates that this is the 6502 or 65C12 BBC BASIC ROM

Byte &8007 = Pointer to the end of title address [minus &8000]. Therefore, as
the value in this byte is &0E, the end of the title is &800E.
Byte &8008 = Internal version number of the ROM = &4 indicating BASIC 4.

Bytes &8009 to &800E contain the title of the ROM ("BASIC"). Ending with a binary 0.
This binary zero (&800E) is the address pointed to by the byte in &8007.
Bytes &800F to &8011 contain the copyright symbol -> "(C)".
Bytes &8012 to &801E contain the copyright string. Ending with a binary 0.
This string contains "1984 Acorn" + chr$(10) [New Line] + chr$(13) [carriage return].
Bytes &801F to &8022 contain the Tube relocation address (32-bit number).
The Tube relocation address for the BASIC ROM is &00008000.

BASIC has no service entry routine.

On entry to the BASIC language, the title string ("BASIC") is automatically printed
(on ROM selection, as it is a language ROM).

Also on entry, the error message pointer (&FD, &FE) is setup to point to
the copyright message (as the BASIC command REPORT proves!) starting at byte &800E as
the error message (BRK) string begins with the Error Number (0 in this case!).

Disassembly for the BASIC ROM header routine