Mastering Sideways ROM & RAM - Module 12 - Absolute workspace
-------------------------------------------------------------

  Private workspace is exclusive to the ROM claiming it. Absolute
workspace can also be claimed by paged ROMs. Absolute workspace,
unlike private workspace, is a single block of memory which is shared
between all the ROMs but is only allocated to one ROM at a time.

  Absolute workspace can be claimed in user memory when it runs from
&E00 to the highest address asked for by any of the ROMs. The Master
computer can also claim the alternative paged memory absolute
workspace. Claiming and using paged memory absolute workspace will be
explained after dealing with user memory absolute workspace.

  In order to claim absolute workspace your SWR interpreter has to
intercept the absolute workspace service call. To demonstrate the
absolute workspace service call load the object code generated by the
program TRACE into SWR and press the Break key (Ctrl+Break on the
Master). A list of service calls similar to the one in figure 12.1
will be printed on the screen. Unless you use a BBC B with an Acorn
6502 Second Processor, Acorn DNFS and sideways RAM in socket &0F your
trace will be different from the one in figure 12.1, but in all BBC
computers you will find service call 1, the absolute workspace claim,
somewhere in the list.




 A=19 X=0F Y=FF SPOOL/EXEC file closure
 A=0F X=0F Y=FF Vectors claimed
 A=FF X=0F Y=FF Tube system main init.
 A=01 X=0F Y=0E Abs. workspace claim
 A=02 X=0F Y=17 Private workspace claim
 A=FE X=0F Y=FF Tube system post init.
 A=11 X=0F Y=1F Font implode/explode

Acorn TUBE 6502 64K

 A=03 X=0F Y=08 Auto-boot
Acorn DFS

 A=10 X=0F Y=EE SPOOL/EXEC file closure
 A=0F X=0F Y=30 Vectors claimed
 A=0A X=0F Y=D4 Claim static workspace
BASIC

>

Figure 12.1  Service call trace after Break
-------------------------------------------



  In figure 12.1 you can see that the absolute workspace claim,
service call 1, is made with Y=&0E. This indicates that no absolute
workspace has yet been claimed. If the trace is run from a lower
priority ROM than the Acorn DFS ROM then service call 1 is made with
Y=&17 indicating that 9 pages of absolute workspace are claimed by the
DFS.

  When service call 1 is made the Y register contains the current
upper limit of the absolute workspace. This starts at &0E and each ROM
should compare the value in the Y register with the upper limit of
absolute workspace required by the ROM and, if necessary, increase it
to the level required by the ROM. Your software must never decrement
the Y register when it intercepts either service call 1 or 2.

  In order to use the absolute workspace effectively it is necessary
for your SWR program to claim private workspace as well. One reason
for this is that your ROM will need to keep a flag indicating whether
or not it has control of the absolute workspace. This flag should be
located in private workspace. It is also a good idea to keep a copy of
any vital data from absolute workspace in private workspace where you
can be sure that it will not be corrupted. If you intend to write SWR
software that uses absolute workspace you should think of the absolute
workspace as a temporary extension to the private workspace and not as
a substitute for private workspace.

  When you want your ROM to use the absolute workspace it has to issue
service call &0A to inform the other ROMs of its intention. Service
call &0A is issued by calling Osbyte &8F with X = service type and
Y = argument for service. In this case make X=&0A and Y=&FF. The ROM
is then free to use the absolute workspace and the zero page locations
from &B0 to &CF until its interpreter intercepts a service call &0A
issued by another ROM. The interpreter must intercept service call &0A
which instructs it to release the absolute workspace to the ROM
issuing the call, usually the DFS.

  I have not written an example program which uses absolute workspace
but I have included a outline program, ABSOL, which includes all the
coding you need to write your own.

  ABSOL intercepts service call 1 to claim absolute workspace, service
call 2 to claim private workspace, sevice call &0A to release absolute
workspace, and service call 4 to recognise the new * command *STATIC.
It uses the one-command interpreter (lines 570-780) used in the
earlier modules of the course.

  The example program ABSOL ensures that at least one page of absolute
workspace is available (lines 300-360) and claims one page of private
workspace (lines 410-450). These are the minimum meaningful amounts if
the program is to use absolute workspace.

  The program recognises service call &0A (lines 470-500). After
recognising service call &0A the program would have to copy any vital
data from absolute to private workspace and adjust a flag kept in
private workspace to indicate that it no longer had control of the
absolute workspace.

  When the interpreter recognises the new command *STATIC it issues
service call &0A (lines 800-830) to inform the other ROMs that it
intends to use the absolute workspace. When it has control of the
absolute workspace it can also use the zero page locations from &B0 to
&CF, but it must be prepared to give up control of both these zero
page locations and the absolute workspace when the interpreter
intercepts a service call &0A issued by another ROM.

  Although using absolute workspace is fairly straightforward if you
use this outline program, it is an unnecessary complication if you
really don't need it. You will probably only need to use it if your
program uses large amounts of data for a short time.





   10 REM: ABSOL
   20 MODE7
   30 HIMEM=&3C00
   40 DIM save 50
   50 diff=&8000-HIMEM
   60 address=&A8
   70 comvec=&F2
   80 ROMnumber=&F4
   90 workspace=&DF0
  100 gsread=&FFC5
  110 osbyte=&FFF4
  120 oscli=&FFF7
  130 FOR pass = 0 TO 2 STEP 2
  140 P%=HIMEM
  150 [       OPT pass
  160         BRK
  170         BRK
  180         BRK
  190         JMP service+diff
  200         OPT FNequb(&82)
  210         OPT FNequb((copyright+diff) MOD 256)
  220         BRK
  230 .title
  240         OPT FNequs("STATIC")
  250 .copyright
  260         BRK
  270         OPT FNequs("(C)1987 Gordon Horsington")
  280         BRK
  290 .service
  300         CMP #1
  310         BNE trytwo
  320         CPY #&0F
  330         BCS carryon
  340         INY           \ claim 1 page of abs. workspace
  350 .carryon
  360         RTS
  370 .trytwo
  380         PHA
  390         CMP #2
  400         BNE tryten
  410         TYA
  420         STA workspace,X
  430         INY           \ 256 bytes of private workspace
  440         PLA
  450         RTS
  460 .tryten
  470         CMP #&0A
  480         BNE tryfour
  490                       \ Prepare to release
  500                       \ absolute workspace
  510 .enough
  520         PLA
  530         RTS
  540 .tryfour
  550         CMP #4
  560         BNE enough
  570         TXA
  580         PHA
  590         TYA
  600         PHA
  610         LDX #&FF
  620 .comloop
  630         INX
  640         LDA title+diff,X
  650         BEQ found
  660         LDA (comvec),Y
  670         INY
  680         CMP #ASC(".")
  690         BEQ found
  700         AND #&DF
  710         CMP title+diff,X
  720         BEQ comloop
  730         PLA
  740         TAY
  750         PLA
  760         TAX
  770         PLA
  780         RTS
  790 .found
  800         LDA #&8F
  810         LDX #&0A      \ service call &0A
  820         LDY #&FF
  830         JSR osbyte    \ Claim absolute workspace
  840                       \ Use zero page from &B0 to &CF
  850                       \ and absolute workspace as well
  860                       \ as private workspace.
  870         PLA
  880         PLA
  890         PLA
  900         LDA #0
  910         RTS
  920 .lastbyte
  930 ]
  940 NEXT
  950 INPUT'"Save filename? : "filename$
  960 IF filename$="" END
  970 $save="SAVE "+filename$+" "+STR$~(HIMEM)+" "+STR$~(las
      tbyte)+" FFFF8000 FFFF8000"
  980 X%=save
  990 Y%=X% DIV 256
 1000 *OPT1,2
 1010 CALL oscli
 1020 *OPT1,0
 1030 END
 1040 DEFFNequb(byte)
 1050 ?P%=byte
 1060 P%=P%+1
 1070 =pass
 1080 DEFFNequw(word)
 1090 ?P%=word
 1100 P%?1=word DIV 256
 1110 P%=P%+2
 1120 =pass
 1130 DEFFNequd(double)
 1140 !P%=double
 1150 P%=P%+4
 1160 =pass
 1170 DEFFNequs(string$)
 1180 $P%=string$
 1190 P%=P%+LEN(string$)
 1200 =pass



  In order to use the alternative paged memory absolute workspace on
the Master computer it is necessary to intercept service call &23
instead of service call 1. Your program must not claim user memory
absolute workspace as well as paged memory absolute workspace.

  Paged memory absolute workspace in the Master starts at &C000 and
has an upper limit of &DBFF. The upper limit of the absolute workspace
must not exceed &DBFF under any circumstances.

  If you intend to use paged memory absolute workspace the outline
program ABSOL will need to be modified. The interpreter will no longer
need to intercept service calls 1 and 2 but it will need to intercept
service call &23 to claim paged memory absolute workspace and service
calls &24 and &22 to claim paged memory private workspace. Suitable
modifications for ABSOL are shown in figure 12.2



.service
        CMP #&23      \ paged absolute workspace claim?
        BNE try24     \ branch if no
        CPY #&C1      \ is 1 page already available?
        BCS carryon   \ branch if yes
        INY           \ claim 1 page of absolute workspace
.carryon
        RTS
.try24
        CMP #&24      \ paged private workspace claim?
        BNE try22     \ branch if no
        INY           \ request 1 page of private workspace
        RTS
.try22
        CMP #&22      \ paged private workspace claim?
        BNE try10     \ branch if no
        PHA           \ store accumulator
        TYA           \ most sig. byte of start address
        STA &DF0,X    \ store start of private workspace
        PLA           \ restore accumulator
        RTS
.try10


Figure 12.2 Modificatins needed for the Master
----------------------------------------------


  In Module 11 I explained how to switch the paged workspace memory in
and out of the main memory map. It is necessary to use the same
techniques described in Module 11 when using paged memory absolute
workspace.

  Before attempting to read or write the contents of the paged memory
absolute workspace, the SWR program must first set bit 3 of the paged
memory select register at &FE34 to switch the paged memory into the
main memory map. To set bit 3 of &FE34 load the accumulator with &08
(0000 1000 binary), and use TSB &FE34.

  To switch the paged memory absolute workspace out of the main memory
map it is necessary to clear bit 3 of the paged memory select
register. To clear bit 3 of &FE34 load the accumulator with &08 (0000
1000 binary), and use TRB &FE34. If you use the subroutines illustrated in
figure 12.3 the paged memory can be switched in and out of the main
memory map without altering any of the other bits in the paged memory
select register.




         JSR pagein      \ switch paged workspace in
         LDA &C000       \ Read first byte of absolute workspace
         JSR pageout     \ switch paged workspace out
          .
          .
          .
.pagein
         PHA
         LDA #8          \ 0000 1000 binary
         TSB &FE34       \ set bit 3 of &FE34
         PLA             \ restore accumulator
         RTS
.pageout
         PHA
         LDA #&08        \ 0000 1000 binary
         TSB &FE34       \ clear bit 3 of &FE34
         PLA             \ restore accumulator
         RTS


Figure 12.3  Reading paged absolute workspace on the Master.
-----------------------------------------------------------

  Figure 12.3 illustrates how the two subroutines, pagein and pageout,
should be used to switch the paged memory in and out of the main
memory map when reading the contents of the paged memory absolute
workspace in the Master series computers. Unlike private workspace,
absolute workspace always has a fixed start address and can be
addressed directly.

  After switching the paged memory into the main memory map you must
be very careful about using MOS subroutines because the bottom 8K of
the MOS has been replaced with the paged memory. You would be wise
not to use the MOS at all until it has been restored by calling
pageout.

  The "legal" use of service call 1 has been explained above but a
much more common "illegal" use is also made of this service call.

  Look again at figure 12.1, the list of service calls issued after
pressing the Break key. Some of these calls, including service call 1,
are always issued on the BBC B when the Break key is pressed and also
when the computer is switched on. (Service call 1 is only issued on
Ctrl-Break on the Master but service call &FE can be used instead
because it is always issued when the Break key is pressed on the
Master computer). Service call 1 or service call &FE can be
intercepted to carry out almost any function required at reset if, and
only if, all the registers are preserved.

  Imagine, for example, that you want the video interlace to stay
switched off. Your SWR interpreter could intercept service call 1 (or
&FE), push all the registers on the stack, switch off the interlace,
pull all the registers back off the stack and return control to the
MOS. This idea has been implemented in the program INTLACE.

  Load the object code generated by INTLACE into SWR, press the Break
key and select Mode 6. The interlace will remain switched off until
you select Mode 7 or press Ctrl+Break. It will switch back off when
you next select one of the modes 0-6.

  This type of illegal use of service call 1 (or &FE) will be used
again in Module 22 when designing the software for auto-booting ROM
cartridges is covered. It will be used then to ensure that the ROM
cartridge is always the language booted after a soft reset.



   10 REM: INTLACE
   20 MODE7
   30 HIMEM=&3C00
   40 DIM save 50
   50 diff=&8000-HIMEM
   60 osbyte=&FFF4
   70 oscli=&FFF7
   80 FOR pass = 0 TO 2 STEP 2
   90 P%=HIMEM
  100 [       OPT pass
  110         BRK
  120         BRK
  130         BRK
  140         JMP service+diff
  150         OPT FNequb(&82)
  160         OPT FNequb((copyright+diff) MOD 256)
  170         BRK
  180         OPT FNequs("INTERLACE")
  190 .copyright
  200         BRK
  210         OPT FNequs("(C)1987 Gordon Horsington")
  220         BRK
  230 .service
  240         CMP #&FE
  250         BEQ autoboot
  260         RTS
  270 .autoboot
  280         PHA
  290         TXA
  300         PHA
  310         TYA
  320         PHA
  330         LDA #&90
  340         LDX #&FF
  350         LDY #1
  360         JSR osbyte    \ *TV255,1
  370         PLA
  380         TAY
  390         PLA
  400         TAX
  410         PLA
  420         RTS
  430 .lastbyte
  440 ]
  450 NEXT
  460 INPUT'"Save filename = "filename$
  470 IF filename$="" END
  480 $save="SAVE "+filename$+" "+STR$~(HIMEM)+" "+STR$~(las
      tbyte)+" FFFF8000 FFFF8000"
  490 X%=save
  500 Y%=X% DIV 256
  510 *OPT1,2
  520 CALL oscli
  530 *OPT1,0
  540 END
  550 DEFFNequb(byte)
  560 ?P%=byte
  570 P%=P%+1
  580 =pass
  590 DEFFNequw(word)
  600 ?P%=word
  610 P%?1=word DIV 256
  620 P%=P%+2
  630 =pass
  640 DEFFNequd(double)
  650 !P%=double
  660 P%=P%+4
  670 =pass
  680 DEFFNequs(string$)
  690 $P%=string$
  700 P%=P%+LEN(string$)
  710 =pass