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5.6 Jump Block Interception - by AMSDOS
When AMSDOS is initialised it copies the relevant cassette jumpblock entries into its
own data area. When DISC is selected the cassette jumpblock entries are overwritten
by AMSDOS entries, when TAPE is selected the original cassette entries are restored.
Initially the disc routines are selected.
5.7 Jump Block Re-Interception - by the User
In order to intercept the jumpblock entries the following procedure should be
observed: copy the three bytes from the required jumpblock entry into your own data
area - do not make any assumption as to what these three bytes are. Replace the
jumpblock entry with your own JMP, RST or whatever. When you receive control
restore the jumpblock entry, and CALL it. When you receive control once again save
the jumpblock entry and replace it with your own. This procedure will work no matter
what the jumpblock entry contains.
Note: when intercepting AMSDOS routines the above procedure must be followed.
Merely executing a copy of the jumpblock entry will not work; it must be restored toits
original place in the jumpblock
5.8 Return Parameters
So far as it is possible the AMSDOS routines all have the same interface as their
cassette counterparts, although in some cases tile interpretation of tile return
parameters is different. Errors which are detected by both the cassette and disc
routines are returned carry false, zero false. Errors which are only detected by the disc
routines are returned carry false, zero true. This latter case corresponds to the cassette
routine BREAK condition. In both cases register A contains an error number.
When a routine fails (carry false) it returns a six bit error number in the A register. Bit
7 is set it the error has already been reported to the user. The error numbers are as
follows:
#0E the file is not open as expected.
#0F hard end of file.
#1A soft end of file.
#20 bad command, usually caused by an incorrect filename.
#21 file already exists.
#22 file doesn't exist.
#23 directory is full
#24 disc is full.
#25 disc has been changed with files open on it.
#26 file is read-only.
AMSTRAD Disc Drive & Interface DDI-1 Manual Chapter 5.6