Mastering Sideways ROM & RAM - Module 01 - Introduction
-------------------------------------------------------

  It is possible to write sideways RAM software on a BBC B or B+ 64K
without sideways RAM but, if you use one of these machines, it will
save you a great deal of time blowing and erasing EPROMs if SWR is
available to test the software as it is being developed.

  Adding SWR to the BBC B+ 64K is very straightforward as Acorn
produce a 64K SWR upgrade which can be fitted by Acorn dealers. Don't
attempt to fit it yourself unless you are absolutely sure you have the
skill needed to complete the job.

  Fitting an SWR upgrade to the BBC B is a job most users can attempt
without incuring a dealer's service charge. There are many sideways
RAM and ROM boards available which can be fitted without soldering any
components. You should look carefully at the facilities offered by the
various manufacturers of SWR boards and choose the board that best
suits your needs. I recommend that you consider the following points
when you look for a SWR board.

1.  A mimimum of 16K of sideways RAM is required. 32K or 64K is useful
but not essential. Most boards which use static RAMs do so because the
design is much simpler. There is an old designer's joke that the
difference between static and dynamic RAM is that static RAM works.
Don't let this nonsense prevent you buying a dynamic RAM board.

2.  Physical switching for read and write protection of the SWR is
highly desirable. Read protection will be used to switch out the
offending memory when you load an invalid ROM image which causes the
computer to hang up. Write protection is useful if you want to develop
ROM images that are protected against running in SWR. Software
switching of read and write protection is a poor substitute for real
switches that click.

3.  Look carefully at the way in which flying leads are attached to
the main computer circuit board. Compare the different methods
manufacturers use. Some boards use quite unprofessional design
techniques.

4.  Battery backup of SWR is useful, but not essential.

5.  Many SWR boards have sockets for extra ROMs. The BBC B MOS will
support up to sixteen sideways ROM or RAM banks. The main circuit
board has only four sideways ROM sockets, two of which are taken up
with BASIC and the DFS. The extra sockets on a sideways RAM/ROM board
will be useful if you have more ROMs than ROM sockets.

  There are a number of manufacturers who produce boards which meet
these requirements to a greater or lesser extent. Which manufacturer
you choose is up to you. You can be sure that all the boards produced
by reputable companies will perform adequately but it is not possible
to recommend one board as better in every respect than any other.

  When you have fitted the upgrade board into your computer you might
want to store the programs contained in your ROMs on disc or tape and
then load and run the ROM images in SWR. All Acorn supported languages
will run in unprotected SWR and most ROM images will run in write
protected SWR.

  Where the copyright of the software contained in a ROM is not owned
by Acorn, specific permission must be obtained from the owner of the
copyright before copying it onto disc or tape.

  In Acorn's User Guide (Part no. 408000, Issue no. 2, March 1984,
page 20) it states "... you can keep a library of paged ROMs on disc,
which can be called up in the same way as ROMs installed inside the
BBC Microcomputer. However, in using this facility you must ensure
that you do not infringe the rights of the owner of the copyright of
the program contained in the ROM".

  As long as you do not infringe the rights of the owner of the
copyright of the program, ROMs can be saved onto a library disc and
run in RAM. Some software producers take a less enlightened view than
Acorn take to running their ROMs in SWR and attempt to protect them
against it. Most of this protection does little to deter professional
software pirates but it should be taken as an indicator of the
copyright owner's unwillingness to allow his or her work to be used in
SWR.

  If you want to ensure that your own work will only run in EPROMs and
not in SWR there are a number of techniques you can try. An 8K ROM
image is duplicated from &A000 and one of the methods of protecting 8K
ROM images is to add &2000 to the subroutine and jump addresses.
Another, far less common, method is to prevent the ROM image running
in socket &0F. This socket is commonly used for SWR on ROM boards and
the only way to get these ROM images to run in socket &0F is to
disassemble and modify them. When you have completed this course you
will certainly know what to look for in a ROM disassembly if you
wanted to do this but it is quite a good way of detering most amateur
ROM pilferers.

  I will now begin to explore the programming techniques needed to use
SWR. There are a number of MOS subroutines, memory locations and
registers associated with SWR. Rather than examine them all now I will
consider the calls and locations as they arise throughout the course.
In this module I will look at the paged ROM select register at &FE30,
the paged memory select register at &FE34, the MOS subroutine Osrdrm,
and Osbytes &AA and &AA.

  There are differences in the way in which the paged ROM select
register and the paged memory select register are used in the BBC B,
the BBC B+ and the Master computers. The reason for the differences
can be seen by examining the memory maps of these machines (figures
1.1, 1.2 and 1.3).



                              &FFFF +-------+ &FFFF
                                    !       !  
                                    !  MOS  !
                                    !       !         
    &BFFF --+-------+-------+-------+-------+ &C000
            ! ROM &C! ROM &D! ROM &E! ROM &F!
            !       !       !       !       !
            !       !       !  DFS  ! BASIC !
    &8000 --+-------+-------+-------+-------+ &7FFF
    Up to 16 sideways ROM/RAM banks !       !
                                    !  User !
                                    ! memory!
                                    !       !
                                    !       !
                              &1900 +-------+ Oshwm 
                                    !       !       
                              &0000 +-------+ &0000

Figure 1.1 Memory map of the BBC B
----------------------------------


  The BBC B MOS supports up to sixteen paged ROMs although an
expansion board is required if more than four ROMs are to be used. The
paged ROM select register at &FE30 is a 4 bit write only register
which determines which of the 16 ROMs is switched into the main memory
map. If you want to read the contents of this register you should read
the zero page copy of it at &F4. The most significant nybble of the
paged ROM select register is not used on the BBC B.



                              &FFFF +-------+ &FFFF
                                    !       !  
                                    !  MOS  !
                                    !       !         
    &BFFF --+-------+-------+-------+-------+ &C000
            ! ROM &C! ROM &D! ROM &E! ROM &F+-------+ &AFFF
            !       !       !       !       ! Paged !
            !       !       !  DFS  ! BASIC ! memory! 
    &8000 --+-------+-------+-------+-------+-------+ &8000
    Up to 16 sideways ROM/RAM banks !       !       !
                                    !  User ! Shadow!
                                    ! memory!  RAM  !
                                    !       !       !
                                    !       +-------+ &3000
                              &1900 +-------+ Oshwm 
                                    !       !       
                              &0000 +-------+ &0000

Figure 1.2 Memory map of the BBC B+
-----------------------------------


  The BBC B+ has a similar memory map to the BBC B but in addition to
the paged ROMs it also supports paged or shadow memory. The paged
memory (shadow RAM) select register at &FE34 is used to determine
whether or not the shadow RAM is used as screen memory. This register
should only be altered using Osbyte &72 (*FX114) or the command
*SHADOW. The BBC B+ has a "spare" 12K of paged memory from &8000 to
&AFFF. This can be switched in or out of the main memory map using the
most significant bit of the paged ROM select register at &FE30.
Setting the most significant bit of the paged ROM select register has
no effect on the BBC B but on the BBC B+ it switches the paged memory
from &8000 to &AFFF into the main memory map. BBC Telesoftware has
broadcast a program that uses this technique to allow BBC B+ 64K
computer users to load and run 8K service ROMs in this area of paged
memory.

  The Master computer uses a slightly different memory map to the B
series. The "spare" 12K of paged memory has now been split into two
areas, one from &8000 to &8FFF and the other from &C000 to &DFFF. The
paged RAM from &8000 to &8FFF is used by the MOS as a soft key buffer,
character font, and VDU workspace. The paged RAM from &C000 to &DCFF
is available as paged ROM workspace and the paged RAM from &DD00 to
&DFFF is used as MOS workspace.




                              &FFFF +-------+ &FFFF
                                    !       !  
                                    !  MOS  +-------+ &DFFF
                                    !       !Wkspace! 
    &BFFF --+-------+-------+-------+-------+-------+ &C000
            ! ROM &C! ROM &D! ROM &E! ROM &F!
            !       !       !       !       !
            ! BASIC ! ADFS  ! View  ! Term. +-------+ &8FFF
    &8000 --+-------+-------+-------+-------+-------+ &8000
    Up to 16 Sideways ROM/RAM banks !       !       !
                                    !  User ! Shadow!
                                    ! memory!  RAM  !
                                    !       !       !
                                    !       +-------+ &3000
                                    !       !
                              &0E00 +-------+ Oshwm 
                              &0000 +-------+ &0000

Figure 1.3 Memory map of the BBC Master
---------------------------------------


  Paged ROM workspace on the BBC B and B+ is just below the user
memory. The BBC B DFS claims 2.75K of workspace from &0E00 to &1900.
This workspace can be in paged memory on the Master series allowing
Oshwm to stay at &0E00 with all filing systems. Bit 3 of the paged
memory select register at &FE34 is used to overlay the paged ROM
workspace onto the Master MOS so that it appears above the paged ROM
using it. Bit 3 of the paged memory select register can be toggled
directly but you must not directly alter any other bit in this
register. This topic will be covered in more detail in modules 11 and
12 which deal with paged ROM workspace.

  The use of the least significant nybble of the paged ROM select
register at is common to all BBC micros. The operating system uses
this register, and the zero page copy of it at &F4, to keep track of
which paged ROM is being used at any time. If you want to alter this
register directly you can only do so from machine code. If you alter
it from BASIC or any other language ROM you will cause the computer to
hang up.

  The order in which the paged ROM select register and its copy at &F4
are altered is important. You must store the same number in both
locations and alter &F4 first. Immediatly after altering &F4 you must
store the same value in &FE30. For example:

  LDA #&0A
  STA &F4
  STA &FE30

  This code will switch bank &0A into the main memory map. If you
reverse the order of the last two instructions you will find that 99%
of the time everything works properly but, sooner or later, an
interupt occuring between the two store instructions will cause to
computer to hang up. Switching a paged ROM into the main memory map in
this way can be quite useful if you want to transfer the contents of a
ROM into user memory or transfer the contents of user memory into a
particular sideways RAM.

  There is an official way to read a byte of memory from any paged
ROM. This uses the operating system call Osrdrm at &FFB9 with the
relevant paged ROM number in the Y register and the address of the
byte in the paged ROM in locations &F6 and &F7 (the paged ROM
pointers). The value of the byte read by Osrdrm is returned in the
accumulator. The X, Y and status registers will be undefined after the
call.

  Both these techniques are demonstrated in the program READROM. When
the program is running it uses Osrdrm to read the title string in
every ROM socket that has an active ROM image and then pROMpts the
user to choose which ROM number, from 0-15, to switch into the main
memory map. The chosen ROM is switched in and copied from &8000 to
&3C00 and then switched back out again. The ROM image can then be
examined and, if required, saved to the currently active filing
system. Although some of the program is in BASIC the switching and
copying can only be done in machine code.

  The program uses Osbyte &72 to find the start of the ROM information
table. This call returns the origin of a 16 byte table containing one
byte per paged ROM. Each byte contains the ROM type byte contained in
location &8006 of the ROM or contains zero if an active ROM is not
present. ROM type bytes are an important part of the ROM header and
will be dealt with in more detail when ROM headers are considered in
module 2.

  Chain the program READROM and satisfy yourself that it works
properly. It must run in the I/O processor and will not work in a
second processor.

  There are two machine code routines used in the program. The first
(lines 200-430) is called once for each possible ROM number from 0 to
15. It looks up the address of the start of the ROM information table
with Osbyte &72 (lines 210-240) and then uses the number of the ROM as
an offset in the Y register to see if there is a valid ROM in the
socket being considered (line 280). If there is a valid ROM image
present the routine reads the title of the ROM using Osrdrm and
displays it on the screen (lines 340-430). The ROM title always starts
at &8009 in a valid ROM image and always ends with a binary zero.

  The second machine code routine is in lines 440-700. This stores the
current ROM number (lines 450-460) before switching the chosen ROM
into the main memory map (lines 470-490). It then copies the chosen
ROM image into user memory (lines 500-660), and finally switches the
original ROM back into the main memory map (lines 670-690).

  The BASIC part of the program is mainly concerned with displaying
the copy of the ROM image on the screen (lines 860-990) and, if
required, with saving a copy of the ROM image onto disc or tape
(PROCsave, lines 1000-1190). Before it makes a copy of the ROM image
onto disc the program compares the first &100 bytes of the image with
&100 bytes 8K higher in memory (lines 1090-1110). If every pair of
&100 bytes is identical the ROM is recognised as an 8K image and only
the first &2000 bytes are saved.

  The program READROM, as well as illustrating the techniques
described above, will be useful in later modules when you will want to
examine or save the contents of sideways RAM banks. The program can be
modified to produce another program to load ROM images into any
sideways RAM bank, including the paged memory bank of the BBC B+. This
has been left as an exercise for you to complete.



   10 REM: READROM
   20 MODE7
   30 HIMEM=&3C00
   40 DIM mcode &100
   50 select=&FE30
   60 osrdrm=&FFB9
   70 osasci=&FFE3
   80 osbyte=&FFF4
   90 oscli=&FFF7
  100 copy=&A8
  110 temp=&A9
  120 read=&AA
  130 write=&74
  140 ROMnumber=&F4
  150 ROMpoint=&F6
  160 ON ERROR PRINT : END
  170 FOR pass=0 TO 2 STEP 2
  180 P%=mcode
  190 [       OPT pass
  200 .number
  210         LDA #&AA
  220         LDX #0
  230         LDY #&FF
  240         JSR osbyte    \ Find ROM table
  250         STX read
  260         STY read+1
  270         LDY copy
  280         LDA (read),Y
  290         BEQ finish
  300         LDA #8
  310         STA ROMpoint
  320         LDA #&80
  330         STA ROMpoint+1
  340 .readname
  350         INC ROMpoint
  360         LDY copy
  370         JSR osrdrm
  380         CMP #ASC(" ")
  390         BCC finish
  400         JSR osasci
  410         JMP readname
  420 .finish
  430         RTS
  440 .move
  450         LDA ROMnumber
  460         STA temp
  470         LDA copy
  480         STA ROMnumber
  490         STA select
  500         LDA #&80
  510         STA read+1
  520         LDA #&3C
  530         STA write+1
  540         LDY #0
  550         STY read
  560         STY write
  570 .relocate
  580         LDA (read),Y
  590         STA (write),Y
  600         INY
  610         BNE relocate
  620         INC read+1
  630         INC write+1
  640         LDA read+1
  650         CMP #&C0
  660         BNE relocate
  670         LDA temp
  680         STA ROMnumber
  690         STA select
  700         RTS
  710 ]
  720 NEXT
  730 FOR socket=0 TO 15
  740 ?copy=socket
  750 IF socket<10 PRINT;" ";
  760 PRINT;socket;" ";
  770 CALL number
  780 PRINT
  790 NEXT
  800 VDU14
  810 REPEAT
  820 INPUT'"Which ROM &(0-15) "socket
  830 UNTIL socket >= 0 AND socket < 16
  840 ?copy=socket
  850 CALL move
  860 PRINT'"<SHIFT> to scroll, <ESCAPE> to exit"
  870 ON ERROR PROCsave
  880 FOR block=&3C00 TO &7BFF STEP 8
  890 PRINT';~block+&4400;"  ";
  900 FOR memory=0 TO 7
  910 byte=block?memory
  920 IF byte<16 VDU48
  930 PRINT;~byte;" ";
  940 NEXT
  950 FOR memory=0 TO 7
  960 byte=memory?block
  970 IF byte>31 AND byte<127 VDUbyte ELSE VDU46
  980 NEXT
  990 NEXT
 1000 PROCsave
 1010 END
 1020 DEFPROCsave
 1030 ON ERROR PRINT : END
 1040 VDU15
 1050 INPUT''"Save ROM? (Y/N) "yes$
 1060 REPEAT
 1070 IF yes$="Y" INPUT"Filename? : "filename$ ELSE INPUT"An
      other? (Y/N) "yes$ : IF yes$<>"Y" END ELSE RUN
 1080 size=FALSE
 1090 FOR block=&3C00 TO &3D00
 1100 IF ?block <> ?(block+&2000) size= TRUE
 1110 NEXT
 1120 IF size=TRUE size=&4000 ELSE size=&2000
 1130 $mcode="SAVE "+filename$+" 3C00+"+STR$~(size)+" FFFF80
      00 FFFF8000"
 1140 X%=mcode
 1150 Y%=X% DIV 256
 1160 CALL oscli
 1170 yes$=""
 1180 UNTIL FALSE
 1190 ENDPROC