Difference between revisions of "Relocating variables on the 6502 Second Processor"
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[[Category:BASIC]] | [[Category:BASIC]] | ||
A 6502 second processor gives you an extra 64K of memory, but not all | A 6502 second processor gives you an extra 64K of memory, but not all | ||
− | programs take advantage of this. | + | 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 | + | assembled to run at &B800 giving you memory from &800 to &B800 giving 44K |
− | available. | + | 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. | + | 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. | |
− | wasted. | ||
FFFF +-----------+ FFFF +-----------+ | FFFF +-----------+ FFFF +-----------+ | ||
Line 29: | Line 28: | ||
The second processor introductory guide suggests putting LOMEM=&C000 and | The second processor introductory guide suggests putting LOMEM=&C000 and | ||
− | storing variables above BASIC. | + | storing variables above BASIC. This works perfectly well, but it implies |
− | that your program is always going to be massively larger than the | + | 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 | |
− | variables. What would be more useful it to put the BASIC program above | + | be more useful it to put the BASIC program above BASIC at &C000 and put the |
− | BASIC at &C000 and put the variables in &800-&8000, giving more space for | + | variables in &800-&8000, giving more space for variables than for the |
− | variables than for the program. | + | program. |
FFFF +-----------+ FFFF +-----------+ | FFFF +-----------+ FFFF +-----------+ | ||
Line 58: | Line 57: | ||
PAGE=&C000:LOMEM=&800 | PAGE=&C000:LOMEM=&800 | ||
− | Unfortunately, it is not quite that simple. | + | 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 | |
− | to do something as simple as listing the program. | + | something as simple as listing the program. Also, you can't just stick a |
− | stick a PAGE move at the start of a program. | + | PAGE move at the start of a program. The program has to be moved to the new |
− | to the new PAGE position. | + | PAGE position. The easiest way of doing this is to load it again. This then |
− | again. | + | raises the problem of knowing what the program was saved as. |
− | as. | ||
Most programs that reload themselves like this hard-wire the name into the | Most programs that reload themselves like this hard-wire the name into the | ||
Line 75: | Line 73: | ||
CHAIN "$.Users.Jim.Progs1.MyName" | CHAIN "$.Users.Jim.Progs1.MyName" | ||
− | Fortunately, there's a way of finding this out. | + | 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 | |
− | program, we can do something like: | + | can do something like: |
CHAIN $&600 | CHAIN $&600 | ||
Line 89: | Line 87: | ||
below its code. | below its code. | ||
− | To start with, we need to see if we are running 6502 | + | 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 | + | processor, and change PAGE, HIMEM and LOMEM to suit and reload if necessary. |
− | necessary. HIMEM cannot be changed inside a PROCedure or FuNction as you | + | HIMEM cannot be changed inside a PROCedure or FuNction as you lose the |
− | lose the return address, so the best thing to do is to return a value to | + | return address, so the best thing to do is to return a value to set HIMEM to |
− | set HIMEM to at the start of the program: | + | at the start of the program: |
HIMEM=FNhimem0 | HIMEM=FNhimem0 | ||
If the program needs to end with the program still in a condition where it | 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, | + | 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: | |
DEFPROCend:HIMEM=FNhimem1:END | DEFPROCend:HIMEM=FNhimem1:END | ||
Line 112: | Line 110: | ||
As LOMEM will not be at the end of the program, TOP should not be used to | 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 | + | find out how much memory is available - this is sloppy programming practice |
− | + | anyway. The following should be used in all cases: | |
maxmem%=HIMEM-LOMEM | maxmem%=HIMEM-LOMEM | ||
− | Now that the structure of the program has been described, here are the | + | Now that the structure of the program has been described, here are the three |
− | + | support functions. They are in the 'HiBASIC' BASIC Library. | |
==FNhimem0== | ==FNhimem0== | ||
Called to initialise the memory arrangement and reload the program if | Called to initialise the memory arrangement and reload the program if | ||
− | necessary. | + | necessary. This function will normally actually be called twice on the |
− | second processor, first with PAGE set to &800 when it reloads the program | + | 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. | |
− | re-arranged. | ||
DEFFNhimem0:A%=130:IF((USR&FFF4)AND&FFFF00)=&FFFF00:=HIMEM | DEFFNhimem0:A%=130:IF((USR&FFF4)AND&FFFF00)=&FFFF00:=HIMEM | ||
− | REM If | + | REM If running in I/O memory, just return current HIMEM |
IF?&FFF7<>&6C OR HIMEM=&B800:=HIMEM | IF?&FFF7<>&6C OR HIMEM=&B800:=HIMEM | ||
REM If not running on a 6502, or we are running HiBASIC, | REM If not running on a 6502, or we are running HiBASIC, | ||
Line 142: | Line 139: | ||
==FNhimem1== | ==FNhimem1== | ||
− | Called to ensure HIMEM is above the program in memory. | + | 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. | |
DEFFNhimem1:IFHIMEM<PAGE:=&F800 ELSE =HIMEM | DEFFNhimem1:IFHIMEM<PAGE:=&F800 ELSE =HIMEM | ||
Line 152: | Line 149: | ||
==FNhimem2== | ==FNhimem2== | ||
Called whenever you want the memory arrangement returned to 'normal', i.e. | Called whenever you want the memory arrangement returned to 'normal', i.e. | ||
− | to chain another program at the normal memory position. | + | to chain another program at the normal memory position. This program will be |
− | + | loaded over whatever data is in the variable area. | |
DEFFNhimem2:IFHIMEM<PAGE:PAGE=&800:=HIMEM ELSE =HIMEM | DEFFNhimem2:IFHIMEM<PAGE:PAGE=&800:=HIMEM ELSE =HIMEM | ||
Line 162: | Line 159: | ||
I have tested these functions on the following BASICs: | I have tested these functions on the following BASICs: | ||
− | BASIC 1, BASIC 2, BASIC 4 (Master), BASIC 40 (Compact), BASIC | + | BASIC 1, BASIC 2, BASIC 4 (Master), BASIC 40 (Compact), Z80 BASIC, |
BASIC 5 (Archimedes) | BASIC 5 (Archimedes) | ||
and on the following hardware: | and on the following hardware: | ||
− | BBC B | + | BBC B I/O, BBC B+6502Tube, BBC B+Z80Tube, Master Compact, |
Master 128 i/o, Master 128+6502Tube, !Z80Tube, !65Host, !65Tube, | Master 128 i/o, Master 128+6502Tube, !Z80Tube, !65Host, !65Tube, | ||
− | + | RISC OS | |
==Demonstration Program== | ==Demonstration Program== | ||
The accompanying program HiDemo (listed below) shows these functions in | The accompanying program HiDemo (listed below) shows these functions in | ||
− | operation. | + | 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 > HiDemo | ||
Line 181: | Line 178: | ||
: | : | ||
HIMEM=FNhimem0 | HIMEM=FNhimem0 | ||
− | + | ON ERROR IF FNerr:PROCend | |
− | PRINT"Program is at &";~PAGE | + | PRINT "Program is at &";~PAGE |
− | PRINT"Data space is at &";~LOMEM;" to &";~HIMEM | + | PRINT "Data space is at &";~LOMEM;" to &";~HIMEM |
− | PRINT"Available memory &";~HIMEM-LOMEM;" (";HIMEM-LOMEM;" bytes)" | + | PRINT "Available memory &";~HIMEM-LOMEM;" (";HIMEM-LOMEM;" bytes)" |
− | + | REPEAT UNTIL FNmenu:PROCend:END | |
: | : | ||
DEFPROCend:HIMEM=FNhimem1:END | DEFPROCend:HIMEM=FNhimem1:END | ||
Line 199: | Line 196: | ||
: | : | ||
DEFFNmenu | DEFFNmenu | ||
− | PRINT"M: Return to 8BS Menu" | + | PRINT "M: Return to 8BS Menu" |
− | PRINT"H: Chain a program in high memory" | + | PRINT "H: Chain a program in high memory" |
− | PRINT"L: Chain a program in low memory" | + | PRINT "L: Chain a program in low memory" |
− | PRINT"Q: Quit" | + | PRINT "Q: Quit" |
: | : | ||
REPEAT | REPEAT | ||
− | A$=GET$: | + | A$=GET$:IF A$="*":REPEAT INPUTLINE"*"A$:OSCLI A$:UNTIL A$="":PRINT":";:A$="*" |
− | A$=CHR$(ASCA$AND(&DF OR(A$<"`"))): | + | A$=CHR$(ASCA$AND(&DF OR(A$<"`"))):UNTIL INSTR("MHLQ",A$):PRINT A$ |
: | : | ||
− | + | 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$ | |
: | : | ||
=A$="Q" | =A$="Q" | ||
This also demonstrates a good way of prompting for and getting a key-press | This also demonstrates a good way of prompting for and getting a key-press | ||
− | from a menu while also allowing multiple '*' commands. | + | from a menu while also allowing multiple '*' commands. It can get very |
− | annoying when programs that only allow you to issue one '*' command and | + | 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 | |
− | means you can't do something like '*.' to see what's there, then '*Info | + | can't do something like '*.' to see what's there, then '*Info xxxx' on |
− | xxxx' on something. | + | something. |
'''By JGH, Nov-1997''' | '''By JGH, Nov-1997''' |
Latest revision as of 01:09, 31 January 2016
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:
PAGE=&C000:LOMEM=&800
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 $&600
as long as we haven't done anything to disturb the string buffer.
So, bearing this in mind, we can construct a program to hold its variables below its code.
To start with, we need to see if we are running 6502 BASIC on a second processor, and change PAGE, HIMEM and LOMEM to suit and reload if necessary. HIMEM cannot be changed inside a PROCedure or FuNction as you lose the return address, so the best thing to do is to return a value to set HIMEM to at the start of the program:
HIMEM=FNhimem0
If the program needs to end with the program still in a condition where it can be LISTed, or you want to load another program in the same space, HIMEM needs to be set above PAGE otherwise 'No room' errors will occur:
DEFPROCend:HIMEM=FNhimem1:END
and
HIMEM=FNhimem1:CHAIN "Program"
And finally, you may want to exit or chain a program into the 'normal' memory arrangement:
HIMEM=FNhimem2:CHAIN "Program"
As LOMEM will not be at the end of the program, TOP should not be used to find out how much memory is available - this is sloppy programming practice anyway. The following should be used in all cases:
maxmem%=HIMEM-LOMEM
Now that the structure of the program has been described, here are the three support functions. They are in the 'HiBASIC' BASIC Library.
FNhimem0
Called to initialise the memory arrangement and reload the program if necessary. This function will normally actually be called twice on the second processor, first with PAGE set to &800 when it reloads the program to &C000, and then with PAGE set to &C000 when LOMEM and HIMEM are re-arranged.
DEFFNhimem0:A%=130:IF((USR&FFF4)AND&FFFF00)=&FFFF00:=HIMEM REM If running in I/O memory, just return current HIMEM IF?&FFF7<>&6C OR HIMEM=&B800:=HIMEM REM If not running on a 6502, or we are running HiBASIC, REM return current HIMEM IFPAGE=&C000:LOMEM=&800:=&8000 REM If we've just been loaded above BASIC, set LOMEM to REM bottom of memory and return to set HIMEM to bottom of REM BASIC ROM code PAGE=&C000:HIMEM=&F800:CHAIN$&600 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
FNhimem1
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.
DEFFNhimem1:IFHIMEM<PAGE:=&F800 ELSE =HIMEM REM If the program is above the variables, return HIMEM REM above the program, otherwise return the current HIMEM
FNhimem2
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.
DEFFNhimem2:IFHIMEM<PAGE:PAGE=&800:=HIMEM ELSE =HIMEM 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, RISC OS
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 : HIMEM=FNhimem0 ON ERROR IF FNerr:PROCend PRINT "Program is at &";~PAGE PRINT "Data space is at &";~LOMEM;" to &";~HIMEM PRINT "Available memory &";~HIMEM-LOMEM;" (";HIMEM-LOMEM;" bytes)" REPEAT UNTIL FNmenu:PROCend:END : DEFPROCend:HIMEM=FNhimem1:END : DEFFNerr:REPORT:PRINT:=INKEY-1 : DEFFNhimem0:A%=130:IF((USR&FFF4)AND&FFFF00)=&FFFF00:=HIMEM IF?&FFF7<>&6C OR HIMEM=&B800:=HIMEM IFPAGE=&C000:LOMEM=&800:=&8000 PAGE=&C000:HIMEM=&F800:CHAIN$&600 DEFFNhimem1:IFHIMEM<PAGE:=&F800 ELSE =HIMEM DEFFNhimem2:IFHIMEM<PAGE:PAGE=&800:=HIMEM ELSE =HIMEM : DEFFNmenu PRINT "M: Return to 8BS Menu" PRINT "H: Chain a program in high memory" PRINT "L: Chain a program in low memory" PRINT "Q: Quit" : REPEAT A$=GET$:IF A$="*":REPEAT INPUTLINE"*"A$:OSCLI A$:UNTIL A$="":PRINT":";:A$="*" A$=CHR$(ASCA$AND(&DF OR(A$<"`"))):UNTIL INSTR("MHLQ",A$):PRINT A$ : 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$ : =A$="Q"
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