The GetNumberFromText subroutine - Revs on the BBC Micro

       Name: GetNumberFromText                                       [Show more]
       Type: Subroutine
   Category: Text
    Summary: Convert a two-digit string into a number
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * GetNumberInput calls GetNumberFromText


 Arguments:

   T                    The first digit of the number, as text

   U                    The second digit of the number, as text


 Returns:

   A                    The numerical value of the number

   C flag               The status of the conversion:

                          * Clear if the string is a valid number and A <= 40

                          * Set if string is not a valid number, or A > 40


.GetNumberFromText

 LDA T                  \ Set A to the character containing the first digit

 CMP #' '               \ If the first digit is not a space, skip the following
 BNE tnum1              \ instruction

 LDA #'0'               \ The first digit is a space, so convert it to a "0"

.tnum1

 SEC                    \ Subtract the ASCII value for "0" to get the numerical
 SBC #'0'               \ value of the first digit into A

 CMP #10                \ If the value of the first digit is greater than 10,
 BCS tnum2              \ then this is not a valid number, so jump to tnum2 to
                        \ return from the subroutine with the C flag set, to
                        \ indicate an error

 STA T                  \ Set T = the value of the first digit

 LDX U                  \ Set X to the character containing the second digit

 CPX #' '               \ If the second digit is a space, then jump to tnum2 to
 CLC                    \ return from the subroutine with the value of the first
 BEQ tnum2              \ digit in A and the C flag clear, to indicate success

 ASL A                  \ Set T = (A << 2 + T) << 1
 ASL A                  \       = (A * 4 + A) * 2
 ADC T                  \       = 10 * A
 ASL A                  \
 STA T                  \ So T contains 10 * the numerical value of the first
                        \ digit

 TXA                    \ Set A to the character containing the second digit

 SEC                    \ Subtract the ASCII value for "0" to get the numerical
 SBC #'0'               \ value of the second digit

 CMP #10                \ If the value of the second digit is greater than 10,
 BCS tnum2              \ then this is not a valid number, so jump to tnum2 to
                        \ return from the subroutine with the C flag set, to
                        \ indicate an error

 ADC T                  \ Set A = A + T
                        \       = the numerical value of the second digit
                        \         + 10 * the numerical value of the first digit
                        \
                        \ which is the numerical value of the two-digit string

 CMP #41                \ If A < 41, clear the C flag, otherwise set it

.tnum2

 RTS                    \ Return from the subroutine