BBC MOS Archeology
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The initial MOS 0.10 had sufficient entry points and API implemented for
BASIC to run - this was essentially just the VDU driver and the CFS. A
lot of the kernel was unimplemented.

The MOS code is split into distinct components, suggesting it was built
from seperate source files and added together:
 &C000 : CHARACTER SET
 &C300 : VDU DRIVERS
 &DB00 : KERNEL
 &F200 : CASSETTE FILING SYSTEM
 &FFA1 : SYSTEM ENTRY BLOCK

MOS entry block included JMP (FSCV) as well as several other entries.
Only 16 vectors, &0200-&021F.
System IRQs not vectored, IRQ1V jumped to after system IRQ handler.
&0200 was EVENTV, not USERV, jumps to 'Bad event'.
Unsure what IRQ2V does, jumps to 'Bad event'.
NMI hardwired to a RTI

No buffer vectors, but MOS entries to access buffers.

OSBYTE generates 'Bad command' error for unimplemented calls instead of
*FX generating this. Only OSBYTE 0-16, 120-145.

A lot of unimplemented calls generate errors instead of the mature MOS
where if a call does nothing, it does nothing, not generate an error.

Three system counters, OSWORD 5/6 reads/writes timer 3.

A lot of memory usage very untidy. Kernel zero page scattered around,
CFS zero page scattered around. Low OSBYTE calls write memory at &0220+.

Different idea of sideways ROM entries.
Sideways ROMs selected Electron-style: LDA #12:STA ROMLATCH:STX ROMLATCH
Startup code 'prods' ROMLATCH, assumes certain settings case Overflow flag
to be set, presumably with something wired to 6502 SO (Set Overflow) pin.

ROM HEADERS:
&8000 - Cold Entry Point - entered on Hard Reset
&8003 - Warm Entry Point - entered on Soft Reset
&8006 - ROM type: b7 = service ROM, called at &8000 on reset
                  b6 = language ROM,
                  b1 = Warm entry exists
&8007 - Offset of copyright message, points to &80xx
&8008
&8009 - ROM title, terminated with &00
&80xx - &00,"(C)" ROM validity string

Some different OSBYTEs:

OSBYTEs:
120 Close SPOOL/EXEC (moved to OSBYTE 119)
121 error (becomes Scan Keyboard)
122 Set LED state (becomes Scan Keyboard)
123 Enter BRK handler (becomes Printer Dormant)
...
136 nothing (becomes *CODE)
137 *MOTOR
138 *CAT - passes string 'CAT' to FSCV if CFS not selected (later insert byte into buffer)
139 *OPT
140 *TAPE
141 Insert byte into buffer (later *ROM)
142 Read function key string (later enter language)
143 Read number of function keys (later issue service call)
144 *TV
145 nothing

Command table includes *DISK, *DISC, *NET, just sets A=4 (disk/disc) and A=3 (net) then
jumps to an error.
Unknown command passed to FSCV with A=1

No *BASIC command, the MOS just enters whatever sideways ROM is present. The layout of
ROMs suggests a future intention to select a ROM using its name as a *command.

No extended function keys. Function keys enter &90-&9A into keyboard buffer, cursor
keys enter &87-&8B into keyboard buffer.