OSBYTE &8F
OSBYTE &8F (143) - Issue SWR Service Request (on exit Y=response to request if appropriate)
X=request number Y=parameter to pass (varies depending on the request number)
Specification
| 6502 | On entry: | On exit: |
| A | =&8F | preserved |
| X | Service request code | =0 service call claimed <>0 service call not claimed |
| Y | Parameter to service call | Result from service call |
- Note
Certain service calls such as &01 and &02 return a result in Y without claiming the call.
Other unclaimed calls preserve Y, provided that all paged ROMs behave correctly.
Side effect
On exit ROMSEL is left set to the current sideways ROM slot number stored in location &F4. J.G.Harston points out that any program may page in a sideways ROM by storing the desired slot number there and immediately calling OSRDSC, but it is also possible to combine a service call issue with a bank switch using the same technique:
\A contains ROM slot number to switch to LDY #&FF \service call parameter = &FF, indicate OSRDSC available LDX #&0E \service call number = &0E, RFS data poll STA &F4 \set new current ROM number LDA #&8F \OSBYTE call number = &8F, issue service call JSR OSBYTE \issue service call \slot number in A on entry is now selected
OSBYTE &8F provides pure ROM paging in OS 1.00, where OSRDSC is not available:
\Y contains ROM slot number to switch to LDX #&00 \service call number = &00, null call LDA #&8F \OSBYTE call number = &8F, issue service call STY &F4 \set new current ROM number JSR OSBYTE \issue service call \slot number in Y on entry is now selected
The OS call encapsulates differences between platforms (particularly on the
Electron) and improves revolving bookcase routines within sideways ROMs,
which no longer need to wait for the chip select lines to stabilise after
switching banks. Note that in such routines, after location &F4 has been
updated, an interrupt service may switch banks early and so all instructions
between STA &F4 and JSR OSBYTE inclusive must be
the same in the source and destination banks.
See Also
Jgharston 21:26, 26 May 2009 (UTC)
