Difference between revisions of "OSRDSC"

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| colspan="2" | '''On entry:'''
 
| colspan="2" | '''On entry:'''
 
|-
 
|-
−
| valign="top" | Y || = Slot number of ROM to be paged in
+
| valign="top" | Y      || = Slot number of ROM to be paged in
 
|-
 
|-
−
| valign="top" | ?&F6 || = LSB of address to be read
+
| valign="top" | ?&F6   || = LSB of address to be read
 
|-
 
|-
−
| valign="top" | ?&F7 || = MSB of address to be read
+
| valign="top" | ?&F7   || = MSB of address to be read
 
|}
 
|}
  
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| colspan="2" | '''On exit:'''
 
| colspan="2" | '''On exit:'''
 
|-
 
|-
−
| valign="top" | A || = contents of address
+
| valign="top" | A      || = contents of address
 
|-
 
|-
−
| valign="top" | X,Y,C || undefined
+
| valign="top" | 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
 
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
+
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
 
the screen.
 
the screen.
 +
 +
===Side effect===
 +
On exit ROMSEL is left set to the current sideways ROM slot number stored in
 +
location &F4; the switch is performed by the internal <tt>STXROM</tt>
 +
routine, which OSRDSC effectively exposes by falling through into it.
 +
 +
J.G.Harston
 +
[https://stardot.org.uk/forums/viewtopic.php?p=345669#p345669 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==
 
==Calling from BBC BASIC==
Line 50: Line 77:
 
* ARM Entry Address: none
 
* ARM Entry Address: none
  
−
==Implementations==
+
==Implementation==
−
The call is absent in MOS 0.10 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&ne;&FF. The entry point exists in MOS 1.20 and is known as '''OSRDRM'''
+
paged ROMs with Y&lt;&gt;&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 to read from paged ROM
+
only. It is documented in MOS 2.00 as '''OSRDSC''' and reads from the specified
−
and the currently displayed screen memory. It is not implemented on non-6502
+
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)

Latest revision as of 20:47, 20 January 2022

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 (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.

Side effect

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

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)

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