Page 212 - Introduction to Microcontrollers Architecture, Programming, and Interfacing of The Motorola 68HC12

7,2 Integer Conversion 189

* CVBTD converts the unsigned contents of D into an equivalent 5-digit ASCII decimal
* number ending at the location passed in X. Register Y is unchanged.
*
CVBTD: PSHX ; Save beginning address of string
LEAK 5, X ; End address of string
PSHX ; Save on stack
LI: LDX #10 ; Divide by 10
LDY #0 ; High 16-bits of dividend (low 16-bits in D)
EDIV ; D is remainder Y is quotient
ADDB #$30 ; ASCII conversion
PULX ; Restore output string pointer
STAB 1 r -X ; Store character, move pointer
PSHX ; Save output string pointer
XGDY ; Remainder to D
CPX 2 , SP ; At end of string?
BNE LI ; Continue
LEAS 4, SP ; Balance stack
RTS ; Exit

a. Using a Loop
* SUBROUTINE CVBTD converts unsigned binary number in D into five
* ASCII decimal digits ending at the location passed in X, using recursion.
*
CVBTD: PSHX ; Save string pointer
LDY #0 ; High dividend
LDX #10 ; Divisor
EDIV ; Unsigned (Y:D) / X -> Y, remainder to D
ADDB #$30 ; Convert remainder to ASCII
PULX ; Get string pointer
STAB 1, -X ; Store in string
TFR Y, D ; Put quotient in D
TBEQ D, LI ; If zero, just exit
BSR CVBTD ; Convert quotient to decimal (recursively)
LI: RTS ; Return

b. Using Recursion

Figure 7.8 Conversion from Binary to Decimal by Division by 10

Looking at the expansion (3) we see that N4 is the quotient of the division of (D) by
(K):(K + 1). If we divide the remainder by (K + 2):(K + 3), we get N3 for the quotient,
and so forth. These quotients are all in binary, so that a conversion to ASCII is also
necessary. The subroutine CVBTD of Figure 7.6 essentially uses this technique, except
that the division is carried out by subtracting the largest possible multiple of each power
of ten which does not result in a carry.