Text: Print2DigitBCD

       Name: Print2DigitBCD                                          [Show more]
       Type: Subroutine
   Category: Text
    Summary: Print a binary coded decimal (BCD) number in the specified format
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * Print4DigitBCD calls Print2DigitBCD
             * PrintDriverTable calls Print2DigitBCD
             * PrintTimer calls Print2DigitBCD
             * UpdateLapTimers calls via Print2DigitBCD-6
             * UpdatePositionInfo calls via Print2DigitBCD-6


 Arguments:

   A                    The number to print (in BCD)

   G                    Flags to control how the number is printed:

                          * Bit 7: clear = do not print leading zeroes
                                   set = print leading zeroes

                          * Bit 6: clear = print second digit
                                   set = do not print second digit


 Returns:

   G                    G is shifted left by two places, so bits 4 and 5 will be
                        used to determine the printing style in the next call to
                        Print2DigitBCD


 Other entry points:

   Print2DigitBCD-6     Print the number at screen coordinate (X, Y), where X
                        is the character column and Y is the pixel row of the
                        bottom of the character


 STX xCursor            \ Set the cursor to (X, Y), so we print the number at
 STY yCursor            \ the specified screen location

.Print2DigitBCD

 PHA                    \ Store A on the stack so we can retrieve it later

 LSR A                  \ Shift the high nibble of A into bits 0-3, so A
 LSR A                  \ contains the first digit of the BCD number
 LSR A
 LSR A

 BNE pnum1              \ If the result is non-zero, jump to pnum1 to print the
                        \ digit in A

                        \ Otherwise the first digit is a zero, which we either
                        \ print as a capital "O" (so it doesn't have a line
                        \ through it), or as a space, depending on the setting
                        \ in G, which controls whether or not to print leading
                        \ zeroes

 LDA #'O'-'0'           \ Set A so we print a capital "O" in pnum1

 BIT G                  \ If bit 7 of G is set, jump to pnum1 to print a capital
 BMI pnum1              \ "O"

 LDA #LO(' '-'0')       \ Otherwise bit 7 of G is clear and we do not print
                        \ leading zeroes, so instead set A so we print a space
                        \ in pnum1

.pnum1

 CLC                    \ Print the high nibble in A as a digit (or, if the high
 ADC #'0'               \ nibble is zero, print a capital "O" or a space, as per
 JSR PrintCharacter     \ the above)

                        \ Now for the second digit

 ASL G                  \ Shift G to the left, so bit 6 is now in bit 7

 PLA                    \ Retrieve the original value of A, which contains the
                        \ BCD number to print

 ASL G                  \ If bit 7 of G is set (i.e. bit 6 of the original G),
 BCS pnum3              \ jump to pnum3 to skip printing the second digit, and
                        \ return from the subroutine

 AND #%00001111         \ Extract the low nibble of the BCD number into A

 BNE pnum2              \ If the low nibble is non-zero, jump to pnum2 to skip
                        \ the following instruction

 LDA #'O'-'0'           \ Set A so we print a capital "O" in pnum2

.pnum2

 CLC                    \ Print the low nibble in A as a digit (or, if the low
 ADC #'0'               \ nibble is zero, print a capital "O")
 JSR PrintCharacter

.pnum3

 RTS                    \ Return from the subroutine

 EQUB &FF               \ This byte appears to be unused