Relocating variables on the 6502 Second Processor

From BeebWiki
Jump to: navigation, search

A 6502 second processor gives you an extra 64K of memory, but not all programs take advantage of this. When BASIC is entered, it is copied over to the second processor and runs from there. If you have HiBASIC, it is assembled to run at &B800 giving you memory from &800 to &B800 giving 44K available. However, this means losing a ROM socket, as you need HiBASIC to run on the second processor and BASIC (assembled to run at &8000) when running with the second processor turned off. Most people end up using BASIC at &8000 as this runs on both sides of the Tube. Unfortunately, this means that the spare memory at &C000 to &F800 in the second processor is wasted.

    FFFF +-----------+                FFFF +-----------+
         | CoPro MOS |                     | CoPro MOS |
    F800 +-----------+                F800 +-----------+
         |  HiBASIC  |                     | Spare RAM |
    B800 +-----------+ HIMEM          C000 +-----------+
         |           |                     |   BASIC   |
         |           |                8000 +-----------+ HIMEM
         |           |                     |           |
         |/\/\/\/\/\/|                     |/\/\/\/\/\/|
         | Variables |                     | Variables |
         +-----------+ LOMEM=TOP           +-----------+ LOMEM=TOP
         |   BASIC   |                     |   BASIC   |
         |  program  |                     |  program  |
    0800 +-----------+ PAGE           0800 +-----------+ PAGE
  6502 CoPro running HiBASIC         6502 CoPro running BASIC

The second processor introductory guide suggests putting LOMEM=&C000 and storing variables above BASIC. This works perfectly well, but it implies that your program is always going to be massively larger than the variables it uses. You have 30K for the program but only 14K for variables. What would be more useful it to put the BASIC program above BASIC at &C000 and put the variables in &800-&8000, giving more space for variables than for the program.

    FFFF +-----------+                FFFF +-----------+
         | CoPro MOS |                     | CoPro MOS |
    F800 +-----------+ HIMEM          F800 +-----------+
         |           |                     |           |
         |           |                     +-----------+ TOP
         |/\/\/\/\/\/|                     |   BASIC   |
         | Variables |                     |  program  |
    B800 +-----------+ LOMEM          C000 +-----------+ PAGE
         |   BASIC   |                     |   BASIC   |
    8000 +-----------+                8000 +-----------+ HIMEM
         |           |                     |           |
         +-----------+ TOP                 |           |
         |   BASIC   |                     |/\/\/\/\/\/|
         |  program  |                     | Variables |
    0800 +-----------+ PAGE           0800 +-----------+ LOMEM
    Variables in high memory           Program in high memory

Initially, it would seem that you would do this with:


Unfortunately, it is not quite that simple. BASIC gets confused if PAGE is higher than HIMEM, you keep getting 'No room' errors whenever you try to do something as simple as listing the program. Also, you can't just stick a PAGE move at the start of a program. The program has to be moved to the new PAGE position. The easiest way of doing this is to load it again. This then raises the problem of knowing what the program was saved as.

Most programs that reload themselves like this hard-wire the name into the program giving something like this:

     PAGE=&xxxx:CHAIN "MyName"

However, this doesn't work if you have called it with something like

     CHAIN "$.Users.Jim.Progs1.MyName"

Fortunately, there's a way of finding this out. When BASIC does a CHAIN, it passes a string to OSFILE to load the program. This string is stored in BASIC's string buffer which is at &0600 in 6502 BASIC, and if you don't do any string operations that file name will still be there. Caution - string operations include printing numbers! So, to reload the current program, we can do something like:

     CHAIN $&600

as long as we haven't done anything to disturb the string buffer.

So, bearing this in mind, we can construct a program to hold its variables below its code.

To start with, we need to see if we are running 6502 BASIC on a second processor, and change PAGE, HIMEM and LOMEM to suit and reload if necessary. HIMEM cannot be changed inside a PROCedure or FuNction as you lose the return address, so the best thing to do is to return a value to set HIMEM to at the start of the program:


If the program needs to end with the program still in a condition where it can be LISTed, or you want to load another program in the same space, HIMEM needs to be set above PAGE otherwise 'No room' errors will occur:



     HIMEM=FNhimem1:CHAIN "Program"

And finally, you may want to exit or chain a program into the 'normal' memory arrangement:

     HIMEM=FNhimem2:CHAIN "Program"

As LOMEM will not be at the end of the program, TOP should not be used to find out how much memory is available - this is sloppy programming practice anyway. The following should be used in all cases:


Now that the structure of the program has been described, here are the three support functions. They are in the 'HiBASIC' BASIC Library.



Called to initialise the memory arrangement and reload the program if necessary. This function will normally actually be called twice on the second processor, first with PAGE set to &800 when it reloads the program to &C000, and then with PAGE set to &C000 when LOMEM and HIMEM are re-arranged.

       REM If running in I/O memory, just return current HIMEM
   IF?&FFF7<>&6C OR HIMEM=&B800:=HIMEM
       REM If not running on a 6502, or we are running HiBASIC,
       REM return current HIMEM
       REM If we've just been loaded above BASIC, set LOMEM to
       REM bottom of memory and return to set HIMEM to bottom of
       REM BASIC ROM code
       REM At this point we must be running in 'normal' memory, so
       REM move PAGE up, put HIMEM above it and reCHAIN the name
       REM stored in the string buffer


Called to ensure HIMEM is above the program in memory. This is needed if you want to END and LIST the program, or load another program over the current program without overwriting any data in the variable space.

       REM If the program is above the variables, return HIMEM
       REM above the program, otherwise return the current HIMEM


Called whenever you want the memory arrangement returned to 'normal', i.e. to chain another program at the normal memory position. This program will be loaded over whatever data is in the variable area.

       REM If the program is above the variables, reset PAGE back
       REM to the bottom of memory, otherwise return the current
       REM HIMEM

I have tested these functions on the following BASICs:

   BASIC 1, BASIC 2, BASIC 4 (Master), BASIC 40 (Compact), Z80 BASIC,
   BASIC 5 (Archimedes)

and on the following hardware:

   BBC B I/O, BBC B+6502Tube, BBC B+Z80Tube, Master Compact,
   Master 128 i/o, Master 128+6502Tube, !Z80Tube, !65Host, !65Tube,

Demonstration Program

The accompanying program HiDemo (listed below) shows these functions in operation. They are also used in real applications in the JGH-PD library in FileIndexer and FormList (on JGH-012) amongst others.

   REM > HiDemo
   REM Demonstrates running above BASIC on 6502Tube
   PRINT "Program is at    &";~PAGE
   PRINT "Data space is at &";~LOMEM;" to &";~HIMEM
   PRINT "Available memory &";~HIMEM-LOMEM;" (";HIMEM-LOMEM;" bytes)"
   IF?&FFF7<>&6C OR HIMEM=&B800:=HIMEM
   PRINT "M: Return to 8BS Menu"
   PRINT "H: Chain a program in high memory"
   PRINT "L: Chain a program in low memory"
   PRINT "Q: Quit"
   IF A$="M":HIMEM=FNhimem2:CHAIN"$.!Boot"
   IF A$="H":INPUT "File to load to high memory: "F$:HIMEM=FNhimem1:CHAINF$
   IF A$="L":INPUT "File to load to low memory: "F$:HIMEM=FNhimem2:CHAINF$

This also demonstrates a good way of prompting for and getting a key-press from a menu while also allowing multiple '*' commands. It can get very annoying when programs that only allow you to issue one '*' command and then force you back to the menu before you can do anything else. That means you can't do something like '*.' to see what's there, then '*Info xxxx' on something.

By JGH, Nov-1997