HEX - Conversion ASCII to HEX.

Syntax:

CALL 4020H
Input:

HL = Start address of a 4-byte hexadecimal number in ASCII format.

Exit:

  • DE = equivalent hexadecimal value (binary)

  • HL = Input address + 4

Registers used:

AF, DE, HL

Error handling:

  • Carry = 0 - no error

  • Carry = 1 - conversion failed

This routine can be used to convert a hexadecimal address input from the keyboard to its binary equivalent.

This routine is used by DOS BASIC, e.g. by the BSAVE command, to interpret and accept the program start address and end address.

Example:

...
  LD HL,ASCII           ; Address ASCII value
  CALL 4020H            ; convert
  JR NC,A1              ; Carry=0? ok, go to A1
  LD A,1                ; Carry=1, "SYNTAX ERROR"
  JP 400EH              ; output via ERROR routine
A1: LD (BIN),DE         ; save binary value
...
ASCII:  DEFM 'A31C'
BIN:    DEFS 2

The character string in the “ASCII” field is converted into the hexadecimal value and stored in the “BIN” field.