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The actual entry codes in the accumulator are:
0
No language present on break. No language ROM is
entered this way, but the Tube language entry point
might be.
1
Normal language start up.
2
Electron only. Request next byte of soft key expansion.
Key number set by call with A=3. Byte out in Y.
3
Electron only. Request length of soft key expansion. Key
number in Y. Length out in Y.
15.1.3 Vectored entry into paged ROMs
As many filing systems are paged ROM resident, a mechanism
has been provided for changing vectors to point into paged
ROMs.
Each vector has a number, n, such that the vector normally
exists at location &0200+2*n. Any vector can be made to point
into a sideways ROM by:
a) Making the main vector at location &0200+2*n point to
&FF00+3*n. This is the operating system's entry point for
processing extended vectors.
b) Ascertaining the extended vector space by issuing OSBYTE
with A=&A8, X=&00, Y=&FF; this returns in X and Y the
address of the extended vector space. Call this address V. (In
OS1.20 V=&0D9F)
c) Setting the extended vector at location V+3*n to:
address in ROM low byte address in ROM high byte ROM
number (held in &F4)
326