Fetches a byte from paged ROM or display memory.
|Y||= Slot number of ROM to be paged in|
|?&F6||= LSB of address to be read|
|?&F7||= MSB of address to be read|
|A||= contents of address|
|X,Y,C||= undefined (all implementations: X=current ROM, Y=&00 (BBC/Electron) Y=preserved (Master))|
Fetches a byte from an address in paged ROM or the current display memory (in main or shadow RAM) and returns it in A. The address to be read is given in locations &F6 (low byte) and &F7 (high byte), and the paged ROM slot to read from is given in Y. If the address lies outside the paged ROM space then the value of Y is irrelevant.
When a paged ROM receives service call &0E, locations &F6 and &F7 are already set up with an appropriate address, and the ROM slot number can be derived from location &F5. If Y=&FF on entry to service call &0E then the operating system provides OSRDRM, so RFS ROMs can quickly serve data bytes on one another's behalf by calling OSRDRM as long as the MOS supports it.
MOS 2.00 introduces a counterpart routine, OSWRSC, to write a byte to the screen.
On exit ROMSEL is left set to the current sideways ROM slot number stored in location &F4; the switch is performed by the internal STXROM routine, which OSRDSC effectively exposes by falling through into it.
J.G.Harston points out that any program may page in a sideways ROM by storing the desired slot number there and immediately calling &FFB9:
STA &F4 \or STX or STY JSR OSRDSC
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.
Calling from BBC BASIC
- BASIC does not call OSRDSC
- BBC BASIC Entry Address: none
- 6502 Entry Address: &FFB9
- Z80 Entry Address: none
- 6809 Entry Address:
- 80x86 Entry Address: none
- 32000 Entry Address: none
- PDP-11 Entry Address: none
- ARM Entry Address: none
The call is absent in MOS 0.10 and MOS 1.00 and service call &0E enters paged ROMs with Y<>&FF. The entry point exists in MOS 1.20 and is known as OSRDRM but is undocumented, and accesses paged ROMs and main memory only. It is documented in MOS 2.00 as OSRDSC and reads from the specified paged ROM/SRAM and the currently displayed screen memory. On the Master series the call reads from whichever screen memory is being accessed by the VDU.
OSRDRM/OSRDSC is not implemented on non-6502 platforms and BBC BASIC does not emulate its entry point.
The function FNrm(), if used on the I/O processor, will read a byte from any banked memory, except the hidden MOS ROM at &FC00-FEFF.
DEFFNrm(!&F6):LOCAL Y%:Y%=?&F8+&40:IFY%>&BF:IF?&F7>&BF:?&F9=0 IF!&F6<0:IF?&F7>&7F OR Y%=&3E:=(USR&FFB9)AND&FF ELSE =?!&F6
The passed address reads the following memory:
Address &0000-&2FFF &3000-&7FFF &8000-&8FFF &9000-&BFFF &C000-&DFFF &E000-&FFFF <&FFxxxxxx main memory main memory current ROM current ROM FS RAM MOS ROM &FF0rxxxx main memory main memory SROM/SRAM r SROM/SRAM r MOS ROM MOS ROM &FF4rxxxx main memory main memory VDU RAM SROM/SRAM r MOS ROM MOS ROM &FF8rxxxx main memory main memory VDU RAM SROM/SRAM r FS RAM MOS ROM &FFFrxxxx main memory main memory SROM/SRAM r SROM/SRAM r MOS ROM MOS ROM &FFFExxxx main memory display mem SROM/SRAM r SROM/SRAM r MOS ROM MOS ROM &FFFFxxxx main memory main memory SROM/SRAM r SROM/SRAM r MOS ROM MOS ROM