Difference between revisions of "OSRDSC"
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Fetches a byte from an address in [[paged ROM]] or the current display | Fetches a byte from an address in [[paged ROM]] or the current display | ||
| − | memory (in main or [[shadow RAM]]) and returns it in A. | + | 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 | 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 slot to read from is given in Y. If the address lies outside the paged | ||
| Line 29: | Line 29: | ||
When a paged ROM receives [[Paged ROM service calls|service call]] &0E, | When a paged ROM receives [[Paged ROM service calls|service call]] &0E, | ||
locations &F6 and &F7 are already set up with an appropriate address, and | 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 then | + | the ROM slot number can be derived from location &F5. If Y=&FF on entry |
| − | the operating system provides OSRDRM, so RFS ROMs can quickly serve data | + | to service call &0E then the operating system provides OSRDRM, so RFS ROMs |
| − | bytes on one another's behalf by calling OSRDRM as long as the MOS supports | + | can quickly serve data bytes on one another's behalf by calling OSRDRM as |
| − | it. | + | long as the MOS supports it. |
MOS 2.00 introduces a counterpart routine, [[OSWRSC]], to write a byte to | MOS 2.00 introduces a counterpart routine, [[OSWRSC]], to write a byte to | ||
| Line 50: | Line 50: | ||
* ARM Entry Address: none | * ARM Entry Address: none | ||
| − | == | + | ==Implementation== |
| − | The call is absent in MOS 1.00 and service call &0E enters paged ROMs with | + | The call is absent in MOS 0.10 and MOS 1.00 and service call &0E enters |
| − | Y& | + | paged ROMs with Y<>&FF. The entry point exists in MOS 1.20 and is known |
| − | but is undocumented, and accesses paged ROMs and main memory only. It is | + | as '''OSRDRM''' but is undocumented, and accesses paged ROMs and main memory |
| − | documented in MOS 2.00 as '''OSRDSC''' and specified | + | only. It is documented in MOS 2.00 as '''OSRDSC''' and reads from the specified |
| − | and the currently displayed screen memory. | + | paged ROM/SRAM and the currently displayed screen memory. On the Master series |
| − | platforms and BBC BASIC does not emulate its entry point. | + | 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. | ||
| + | |||
| + | ==Coding== | ||
| + | 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 | ||
-- [[User:Beardo|beardo]] 23:20, 5 October 2007 (BST) | -- [[User:Beardo|beardo]] 23:20, 5 October 2007 (BST) | ||
| + | [[User:Jgharston|Jgharston]] ([[User talk:Jgharston|talk]]) 04:25, 29 March 2015 (UTC) | ||
Revision as of 05:25, 29 March 2015
Fetches a byte from paged ROM or display memory.
Specification
| On entry: | |
| Y | = Slot number of ROM to be paged in |
| ?&F6 | = LSB of address to be read |
| ?&F7 | = MSB of address to be read |
| On exit: | |
| A | = contents of address |
| X,Y,C | undefined |
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.
Calling from BBC BASIC
- BASIC does not call OSRDSC
Entry points
- 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
Implementation
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.
Coding
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
-- beardo 23:20, 5 October 2007 (BST) Jgharston (talk) 04:25, 29 March 2015 (UTC)
