Decimal to Hex and Back

Converting from hexadecimal to decimal number systems has been the bane of many an aspiring machine code programmer. This listing does the converting for you. When you RUN the program, a prompt asks you for a hex number. You can either type one in to get the decimal equivalent, or press ENTER to flip to the decimal to hex mode , where you can type in a base 10 number to get the hex form.

The program can be used as a foundation for a much more elaborate monitor. Subroutines at lines 1000 and 2000 convert the number held in A$ from hex to decimal or decimal to hex respectively. The program returns from the subroutines with the hex number in A$ and the decimal number in C. An error flag (called ERR) is set to 1 if a faulty input is received.

  10 LET ERR=0: DEF FN A(H$)=CODE H$-48-7*(H$>"@"): LET H$="0123456789ABCDEF"
  20 INPUT "HEX into DEC.?";A$: IF A$="" THEN GO TO 100
  30 GO SUB 1000: REM hex to dec
  35 IF ERR=1 THEN PRINT "INVALID NUMBER": GO TO 1
  40 PRINT A$;" hex=";C;" dec."
  50 GO TO 20
 100 INPUT "DEC into HEX?";A$: IF A$="" THEN GO TO 10
 110 GO SUB 2000: REM dec to hex
 120 GO TO 35
1000 LET C=0: FOR X=1 TO LEN A$: IF CODE A$(X)>=48 AND CODE A$(X)<=70 THEN LET C=C+FN A(A$(LEN A$-X+1))*16↑(X-1): NEXT X: RETURN
1010 LET ERR=1: RETURN
2000 FOR X=1 TO LEN A$: IF CODE A$(X)>47 AND CODE A$(X)<58 THEN NEXT X: GO TO 2010
2005 GO TO 1010
2010 LET C=VAL A$: IF C>65535 THEN GO TO 1010
2015 RANDOMIZE C: IF C=0 THEN RETURN
2020 LET P=23671: LET A$=""
2030 LET N=PEEK P: LET N$=H$(N/16+.5): LET N=N-INT (N/16)*16+.5: LET N$=N$+H$(N): LET A$=A$+N$: IF P=23670 THEN RETURN
2040 LET P=P-1: GO TO 2030