The BBC and Master Computer Public Domain Library
8000 BASIC ROM header
Submitted by Steve Fewell
Description:
The first 3 bytes contain code to jump to the BASIC language entry startup routine.
The next 3 bytes contain code to jump to the BASIC ROM service entry startup routine.
Byte &8006 = ROM Type. &E2 indicates a language & a service ROM (i.e. it has a language entry address
& a service entry address & a second processor relocation address).
ROM Type &E2 [1110 0010] indicates that:
* A Service entry point is present (Bit 7 is set)
* 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 - 0010 indicates that this is 6502 or 65C12 code
Byte &8007 = Pointer to the end of title address [minus &8000]. Therefore, as
the value in this byte is &13, the end of the title is &8013.
Byte &8008 = Internal version number of the ROM = &7.
Bytes &8009 to &800E contain the title of the ROM ("BASIC"). Ending with a binary 0.
Bytes &800F to &8013 contain the release number of the ROM ("4r32"). Ending with a binary 0.
This binary zero (&8013) is the address pointed to by the byte in &8007.
Bytes &8014 to &8016 contain the copyright symbol -> "(C)".
Bytes &8017 to &8023 contain the copyright string. Ending with a binary 0.
This string contains "1988 Acorn" + chr$(10) [New Line] + chr$(13) [carriage return].
Bytes &8024 to &8025 contain the Tube relocation address (16-bit address).
The Tube relocation address for the BASIC ROM is &B800.
Bytes &8026 to &8027 contain a pointer to the bit-map descriptor table.
The Bit-map descriptor table is &8028.
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 &8013 as
the error message (BRK) string begins with the Error Number (0 in this case!).
The Bit-map descriptor table:
This table contains 4 bytes. The first 2 are a pointer to the byte after the bit-map - which is &C000.
The next byte is the relative ROM number that the bit-map is held in - which is &82 (130).
(note: the top bit is set which indicated that the ROM number is relative - not absolute).
So, after ignoring the top bit flag, the ROM number that contains the bit-map is in a slot 2 higher than the
BASIC ROM. So as BASIC is in ROM socket C, this refers to the VIEW ROM (in socket E).
The end of the [MOS 3.50's] VIEW ROM (location &BFFE, which is address &C000 minus 2) contains the required Bit-Map
marker characters: &C0 followed by &DE. Bytes &BFFC and &BFFD contain the length of the bit-map
which is &02D0. Further details of the bit-map are considered beyond the scope of this BASIC guide.
Changes from BASIC IV:
Added Service Address, version number and Bit-map desscriptor table.
Disassembly for the BASIC ROM header routine
| 8000 | L | 076 231 128 | 4C E7 80 | JMP &80E7 BASIC Language Startup routine |
| 8003 | L, | 076 044 128 | 4C 2C 80 | JMP &802C BASIC ROM Service startup address |
| 8006 | 226 | E2 | EQUB &E2 | |
| 8007 | 019 | 13 | EQUB &13 Points to &8013 | |
| 8008 | 007 | 07 | EQUB &7 | |
| 8009 | BASIC | 066 065 083 073 067 | 42 41 53 49 43 | EQUS "BASIC" |
| 800E | 000 | 00 | EQUB &0 | |
| 800F | 4r32 | 052 114 051 050 | 34 72 33 32 | EQUS "4r32" |
| 8013 | 000 | 00 | EQUB &0 | |
| 8014 | (C) | 040 067 041 | 28 43 29 | EQUS "(C)" |
| 8015 | 1988 Acorn | 049 057 056 056 032 065 099 111 114 110 | 31 39 38 38 20 41 63 6F 72 6E | EQUS "1988 Acorn" |
| 8021 | 010 | 0A | EQUB &0A | |
| 8022 | 013 | 0D | EQUB &0D | |
| 8023 | 000 | 00 | EQUB &00 | |
| 8024 | 000 184 | 00 B8 | EQUW &B800 | |
| 8026 | 040 128 | 28 80 | EQUW &8028 | |
| 8028 | 000 192 | 00 C0 | EQUW &C000 | |
| 802A | 130 | 82 | EQUB &82 | |
| 802B | 000 | 00 | EQUB &00 |
