Page 296 - Introduction to Microcontrollers Architecture, Programming, and Interfacing of The Motorola 68HC12

9.3 Conditionai Statements 273

The case statement is a useful alternative to the conditional statement. Consider an
expression like switch(n){ case 1: i=l; break; case 3: i=2; break; case 6:
i=3;break;}. This is compiled into assembly language by calling a subroutine
switch to evaluate the case and providing the cases as addresses below its call, as
shown in Figure 9.9a. This technique is used in Hi Ware's compiler when the cases are,
or are nearly, consecutive numbers. Another technique used in some compilers is to
implement the test using a sequence of CMP A and BNE instructions. Assembly language
in Figure 9.9b implements the same switch statement with branch instructions. This
technique is used in HiWare's compiler when the cases are not consecutive numbers.

* switch(n){case 0:1=1; break; case l:i=2; break; case 2:i=3; break;}
00000867 EC81 LDD 1,SP
00000869 072D BSR switch
0000086B 03 L: DC.B LO-L
0000086C 06 DC.B Ll-L
0000086D 09 DC.B L2-L
5: case 1: i=l; break;
0000086E C601 LO: LDAB #1
00000870 8FC602 CPS #50690 ; is SKIP2 L1:LDAB #2
6: case 3: i=2; break;
00000873 8FC603 CPS #50691 ; is SKIP2 L2:LDAB #3
00000876 6B80 STAB 0,SP
* .. .
00000898 30 switch: PULX
00000899 E6E6 LDAB D,X
0000089B 05E5 JMP B,X
a) Using a Subroutine and Argument List

* switch(n){case l:i=l; break;case 3:i=2;break;case 6: i=3;break;}
LDAA $ 0 8 0 0 ; get switch operand alpha
CMPA #6 ; check for last case
BHI *+2 7 ; branch over the rest of the cases
CMPA #1 ; check for first case
BEQ *+12 ; if found, go to LDAB instruction
CMPA #3 ; check second case
BEQ * +11 ; if so go to middle of first CPS instruction
CMPA #6 ; check last case
BEQ * + 10 ; if so go to middle of second CPS instruction
BRA * -f 13 ; if not matched, skip over cases
LDAB #1 ; this is for the case one
CPS #50690 ; skip, or LDAB #2
CPS #50691 ; skip, or LDAB #3
STAB $ 0 8 01 ; store result in beta
b) Using a Sequence of CMP and conditional branch instruction Pairs

Figure 9.9. Alternative Assembly Language for a Case Statement