Address of a variable

BBC BASIC for Windows implements the ^identifier function (as opposed to the num ^ num operator) to return the address of a variable. For example, ^fred% returns the address where the data for the variable fred% is stored.

This can be implemented on other versions of BBC BASIC with the following code, which is also available in the AddrOf BASIC library.

6502

   DEFFNAddrOf_65:DIM A% 31:P%=A%
   [OPT2:LDA &604:STA &70:LDA &605:STA &71:LDY #0
   LDA &601:STA (&70),Y:INY:LDA &602:STA (&70),Y:INY
   LDA #0:STA (&70),Y:INY:STA (&70),Y:RTS:]:=A%

Z80

   DEFFNAddrOf_Z80:DIM A% 23:P%=A%
   [OPT 2:LD L,(IX+5):LD H,(IX+6):LD A,(IX+2):LD (HL),A
   INC HL:LD A,(IX+3):LD (HL),A:INC HL:LD (HL),0:INC HL
   LD (HL),0:RET:]:=A%

80186

BBC BASIC for Windows implements ^identifier. DOS BASIC does not, and the following code must be used.

   DEFFNAddrOf_86:DIM A% 19:P%=A%
   [OPT 2:MOV AX,DS:[BP+2]:MOV BX,DS:[BP+5]:MOV DS:[BX],AX
   MOV AX,0:MOV DS:[BX+2],AX:RETF:]:=A%

PDP-11

PDP-11 BASIC implements ^identifier natively. The following code can be used for compatibility. Current versions of PDP-11 BASIC does not have an inline assembler, and the code must be created manually.

   DEFFNAddrOf_PDP:DIM A% 15:!A%=&C1D80:A%!4=&81D81:A%!8=&A311009:A%!12=870002:=A%

This creates the following code:

   MOV 12(SP),R0:MOV 8(SP),R1:MOV R0,(R1):CLR 2(R1):RTS PC

ARM

   DEFFNAddrOf_ARM:DIM A% 15
   [OPT2:LDR R0,[R9]:LDR R1,[R9,#8]:STR R1,[R0]:MOV PC,R14:]:=A%

Usage

Once the appropriate FNAddrOf_* function is called to set up the machine code, for instance addrof%=FNAddrOf_65, then the address of a variable can be found with CALL addrof%,variable,result%, leaving the address in result%. result% must be a pre-existing integer variable. CALLing addrof% with fewer than two parameters, or with a non-integer result will have undefined results.

BBC BASIC for Windows allows ^PROCname and ^FNname. This code only supports those variables that can be passed to the CALL command.

After calling, result% holds the address of the variable's data block, as with parameters passed to CALL. This will be the following:

CALL addrof%,num,result%	result% -> 5-byte floating point value
CALL addrof%,num%,result%	result% -> 4-byte integer value
CALL addrof%,str$,result%	result% -> string information block

The string information block is different on different platforms. See CALL for details.

Examples

   REM Print exponent of PI
   num=PI
   CALL addrof%,num,A%
   PRINT "Exponent: ";?A%

   REM Load 4-byte file into a variable
   fred%=0:addr%=0:CALL addrof%,fred%,addr%
   OSCLI "LOAD data "+STR$~A%

   REM Swap byte order
   big%=&12345678
   little%=big%
   CALL addrof%,little%,A%
   tmp%=A%?0:A%?0=A%?3:A%?3=tmp%
   tmp%=A%?1:A%?1=A%?2:A%?2=tmp%

Jgharston 00:06, 5 December 2009 (UTC)