Using high workspace

On the BBC Master there is extra workspace memory at &C000-&DFFF called Hazel. This is available for sideways ROM workspace instead of using memory at &E00 and pushing up BASIC's PAGE. A sideways ROM can use the Hazel workspace by responding to the Hazel workspace service calls &21 to &24. There is space for shared workspace from &C000 upwards and private workspace from &DC00 downwards.

Private workspace in low memory is claimed from &E00 upwards, but private workspace in Hazel is claimed from &DC00 downwards. This gives two results. It means that if not all of the high workspace is claimed the current owner of the shared workspace can use the rest of the workspace up to the bottom of the private workspace reported by Service Call &23. It also means that if more workspace is claimed than is available then a sideways ROM can respond to service call 2 and claim workspace in low memory instead.

As an example, imagine a system with four ROMs each claiming one page of private workspace, and the largest amount of shared memory is eight pages. This will give the following layout:

 DC00 +------------------+
      | ROM 1 workspace  |
 DB00 +------------------+
      | ROM 2 workspace  |
 DA00 +------------------+
      | ROM 3 workspace  |
 D900 +------------------+
      | ROM 4 workspace  |
 D800 +------------------+
      |                  |
 C800 +------------------+ Top of shared workspace
      | Shared workspace |
 C000 +------------------+

The owner of the shared workspace at &C000 can expand its workspace up to &D800.

On the other hand, imagine eight ROMs each claiming four pages of private workspace. This is &2000 bytes, but there is only &1C00 bytes of workspace available without accounting for any shared workspace. In this instance, ROMs have to note that there is not enough high workspace and claim it from low memory instead. This is done by pointing to the overflowed workspace on the high workspace service call, and checking for this on the low workspace service call. This example would give the following memory layout, again with eight pages of shared workspace:

 DC00 +------------------+
      | ROM 4 workspace  |
 D800 +------------------+
      | ROM 5 workspace  |
 D400 +------------------+
      | ROM 6 workspace  |
 D000 +------------------+
      | ROM 7 workspace  |
 CC00 +------------------+
      | ROM 8 workspace  |
 C800 +------------------+ Top of shared workspace
      | Shared workspace |
 C000 +------------------+
      :                  :
      :                  :
 1A00 +------------------+
      | ROM 1 workspace  |
 1600 +------------------+
      | ROM 2 workspace  |
 1200 +------------------+
      | ROM 3 workspace  |
 0E00 +------------------+

The following example code from HADFS claims one page of private workspace and nine pages of shared workspace. The code checks if it is operating on a BBC or a Master. On a BBC it claims low shared and private workspace, on the Master it claims high shared workspace, and claims either high or low private workspace.

The service calls are listed in the order they are called:

 \ -----------------------------------------------------------
 \ SERVICE &24 - State how much private Hazel workspace needed
 \ -----------------------------------------------------------
 .Serv24
 DEY:RTS                           :\ Need one page of private workspace
 :
 \ ----------------------------------------------------------
 \ SERVICE &21 - State how much shared Hazel workspace needed
 \ ----------------------------------------------------------
 .Serv21
 CPY #&C9:BCS P%+4:LDY #&C9:RTS    :\ Need at least nine pages of shared workspace
 :
 \ -------------------------------------------
 \ SERVICE &22 - Claim Hazel private workspace
 \ -------------------------------------------
 .Serv22
 CPY #&DC:BCC P%+4:LDY #&DC        :\ If run out of Hazel, point to top of Hazel
 TYA:STA &DF0,X                    :\ Note location of private workspace
 INY:LDA #&22:RTS                  :\ Claim one page of workspace and return
 :
 \ ------------------------------------------------------
 \ SERVICE 1 - State how much shared low workspace needed
 \ ------------------------------------------------------
 .Serv1
 LDA OSFILE:CMP #&6C:BCC Serv1done :\ Don't claim shared low workspace on Master
 CPY #&17:BCS P%+4:LDY #&17        :\ BBC needs nine pages of shared low workspace
 .Serv1done
 LDA #1:RTS                        :\ Restore and return
 :
 \ ---------------------------------------
 \ SERVICE 2 - Claim private low workspace
 \ ---------------------------------------
 .Serv2
 LDA OSFILE:CMP #&6C:BCS Serv2bbc  :\ Always claim low workspace on BBC
 LDA &DF0,X                        :\ Get workspace pointer
 CMP #&DC:BCC Serv2done            :\ If we haven't run out of Hazel, use it
 .Serv2bbc
 TYA:STA &DF0,X                    :\ Note location of private workspace
 INY                               :\ Use one page of low workspace
 .Serv2done
 \ ROMs often do Break initialisation here
 LDA #2:RTS                        :\ Restore and exit
 :
 \ ------------------------------------------------------
 \ SERVICE &23 - Report bottom of Hazel private workspace
 \ ------------------------------------------------------
 .Serv23
 \ On entry, Y=start of private workspace in Hazel
 \ The ROM can store this value and extend its shared workspace
 \ up to this value when it owns the shared workspace
 LDA #&23:RTS

A consequence of the workspace pointer being able to point to memory at from &0000 upwards or &C000 upwards, the usual method of using bit 7 as an enabled/disabled flag cannot be used. Instead, both bit 7 and bit 6 are used, with %00xxxxxx and %11xxxxxx indiating the ROM is enable with workspace at &0000 or &C000, and %01xxxxxx and %10xxxxxx indicating the ROM is disabled. This can be checked for with the following code in the service handler:

 .Service
 PHA:LDA &DF0,X                      :\ Get workspace flags
 BMI P%+4:EOR #&40                   :\ Toggle bit 6 if bit 7 is zero
 ASL A:BPL ServDisabled:PLA          :\ Exit if the resultant bit 6 is clear
 \
 \ Do the service call dispatch
 \
 .ServDisabled
 PLA:RTS                             :\ Restore service call and return

Low shared workspace can be claimed by any sideways ROM by issuing Service Call &0A. High shared workspace is claimed by Filing System Control call &06, so is normally only claimable by filing systems.

WikiSysop (talk) 13:51, 8 March 2015 (UTC)