Enhanced BASIC, using USR()
USR(<expression[$]>) calls the machine code function pointed to by the user jump vector after evaluating <expression[$]> and placing the result in the first floating accumulator. Once the user function exits, via an RTS, the value in the floating accumulator is passed back to EhBASIC.
Either a numeric value or a string can be passed, and either type can be returned depending on the setting of the data type flag at the end of the user code and the return point (see code examples for details).
It can also be extended by adding parameters to USR() and parsing them from within the routine in the same way that CALL can be extended, just remember to get the value from FAC1 first.
How to – numeric source, numeric result.
First you need to write the code.
; this code demonstrates the use of USR() to quickly calculate the square of a ; byte value. Compare this with doing SQ=A*A or even SQ=A^2. .include BASIC.DIS ; include the BASIC labels file. this allows ; you easy access to the internal routines you ; need to parse the command stream and access ; some of the internals of BASIC. It is usually ; output by the assembler as part of the listing ; or as a separate, optional, file. ; for now we'll put this in the spare RAM at $F400 *= $F400 Square JSR LAB_EVBY ; evaluate byte expression, result in X and FAC1_3 LDA #$00 ; clear A STA FAC1_2 ; clear square low byte (use FAC1 as the workspace) ; (no need to clear the high byte, it gets shifted out) TXA ; copy byte to A LDX #$08 ; set bit count Nextr2bit ASL FAC1_2 ; low byte *2 ROL FAC1_1 ; high byte *2+carry from low ASL A ; shift byte BCC NoSqadd ; don't do add if C = 0 TAY ; save A CLC ; clear carry for add LDA FAC1_3 ; get number ADC FAC1_2 ; add number^2 low byte STA FAC1_2 ; save number^2 low byte LDA #$00 ; clear A ADC FAC1_1 ; add number^2 high byte STA FAC1_1 ; save number^2 high byte TYA ; get A back NoSqadd DEX ; decrement bit count BNE Nextr2bit ; go do next bit LDX #$90 ; set exponent=2^16 (integer) SEC ; set carry for +ve result JMP LAB_STFA ; set exp=X, clearFAC1 mantissa3, normalise & return
Now you need to set up the address for your function. This is done by DOKEing an address into the USR() vector e.g.
DOKE $0B,$F400 ; set the user function address to addr ; $0B - user function vector address ; $F400 - routine address
Finally you need to set the vector in your BASIC program and use that to call the function
. 10 DOKE $0B,$F400 . . . 145 SQ=USR(A) .
How to – numeric source, string result.
As before, first you need to write the code.
; this code demonstrates the use of USR() to generate a string of # characters. ; the length of the required string is the parameter passed. .include BASIC.DIS ; include the BASIC labels file. this allows ; you easy access to the internal routines you ; need to parse the command stream and access ; some of the internals of BASIC. It is usually ; output by the assembler as part of the listing ; or as a separate, optional, file. ; for now we'll put this in the spare RAM at $F400 *= $F400 STRING JSR LAB_EVBY ; evaluate byte expression, result in X and FAC1_3 TXA ; string is byte length BEQ NUL_STRN ; branch if null string JSR LAB_MSSP ; make string space A bytes long A=$AC=length, ; X=$AD=Sutill=ptr low byte, Y=$AE=Sutilh=ptr high byte LDA #"#" ; set character LDY FAC1_3 ; get length SAV_HASH DEY ; decrement bytes to do STA (str_pl),Y ; save byte in string BNE SAV_HASH ; loop if not all done NUL_STRN PLA ; dump return address (return via get value PLA ; from line, this skips the type checking and ; so allows a string result to be returned) JMP LAB_RTST ; check for space on descriptor stack then put ; string address and length on descriptor stack ; & update stack pointers
Now you need to set up the address for your function. This is done by DOKEing an address into the USR() vector e.g.
DOKE $0B,$F400 ; set the user function address to addr ; $0B - user function vector address ; $F400 - routine address
Finally you need to set the vector in your BASIC program and use that to call the function
. 10 DOKE $0B,$F400 . . . 145 HA$=USR(A) .
How to – string source, numeric result.
As before, first you need to write the code.
; this code demonstrates the use of USR() to test a string of characters. ; if all the string is alpha -1 is returned, else 0 is returned. .include BASIC.DIS ; include the BASIC labels file. this allows ; you easy access to the internal routines you ; need to parse the command stream and access ; some of the internals of BASIC. It is usually ; output by the assembler as part of the listing ; or as a separate, optional, file. ; for now we'll put this in the spare RAM at $F400 *= $F400 ALPHA JSR LAB_EVST ; evaluate string TAX ; copy length to X BEQ NOT_ALPH ; branch if null string LDY #$00 ; clear index ALP_LOOP LDA (ut1_pl),Y ; get byte from string JSR LAB_CASC ; is character "a" to "z" (or "A" to "Z") BCC NOT_ALPH ; branch if not alpha INY ; increment index DEX ; decrement count BNE ALP_LOOP ; loop if not all done LDA #$FF ; set for -1 BNE IS_ALPHA ; branch always NOT_ALPH LDA #$00 ; set for 0 IS_ALPHA TAY ; copy byte LDX #$00 ; clear byte STX Dtypef ; clear data type flag, $00=numeric JMP LAB_AYFC ; save & convert integer AY to FAC1 & return
Now you need to set up the address for your function. This is done by DOKEing an address into the USR() vector e.g.
DOKE $0B,$F400 ; set the user function address to addr ; $0B - user function vector address ; $F400 - routine address
Finally you need to set the vector in your BASIC program and use that to call the function
. 10 DOKE $0B,$F400 . . . 145 AL=USR(A$) .
How to – string source, string result.
As before, first you need to write the code.
; this code demonstrates the use of USR() invert the case of a string of ; characters. only alpha characters will be affected. .include BASIC.DIS ; include the BASIC labels file. this allows ; you easy access to the internal routines you ; need to parse the command stream and access ; some of the internals of BASIC. It is usually ; output by the assembler as part of the listing ; or as a separate, optional, file. ; for now we'll put this in the spare RAM at $F400 *= $F400 ALPHA JSR LAB_EVST ; evaluate string STA str_ln ; set string length STX str_pl ; set string pointer low byte STY str_ph ; set string pointer high byte TAX ; copy length to X BEQ NO_STRNG ; branch if null string LDY #$00 ; clear index ALP_LOOP LDA (ut1_pl),Y ; get byte from string JSR LAB_CASC ; is character "a" to "z" (or "A" to "Z") BCC NOT_ALPH ; branch if not alpha EOR #$20 ; toggle case STA (ut1_pl),Y ; save byte back to string NOT_ALPH INY ; increment index DEX ; decrement count BNE ALP_LOOP ; loop if not all done NO_STRNG PLA ; dump return address (return via get value PLA ; from line, this skips the type checking and ; so allows a string result to be returned) JMP LAB_RTST ; check for space on descriptor stack then put ; string address and length on descriptor stack ; & update stack pointers
Now you need to set up the address for your function. This is done by DOKEing an address into the USR() vector e.g.
DOKE $0B,$F400 ; set the user function address to addr ; $0B - user function vector address ; $F400 - routine address
Finally you need to set the vector in your BASIC program and use that to call the function
. 10 DOKE $0B,$F400 . . . 145 A$=USR(A$) .
See also:
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