Page 129 - Introduction to Microcontrollers Architecture, Programming, and Interfacing of The Motorola 68HC12

106 Chapter 4 Assembly Language Programming

*;(;**;};*****.•(;**************
* Get an Opcode
* entry: B is mnemonic opcode character
* exit: A is opcode
* X is unchanged
+
GETOPCD: CLRA ; if exit next, return zero as machine opcode
CMPB #'L' ;Load
BEQ GO1
LDAA #$40 ; if exit next, return $40 as machine opcode
CMPB #'A';Add
BEQ G01 ; if it isn't D, L, or A, it must be S, for Store
LDAA #$80; return $80 as machine opcode
G01: RTS

Figure 4.19. Subroutine to Get the Opcode

FINDLBL (Figure 4.20) begins by setting up X for a loop; the loop searches each
row of the symbol table, which was created in PASS1, until it finds a match. It searches
each row of the symbol table. Because we assume there are no errors in our source code,
it will always succeed in returning the value for the matching label.
GETHEX (Figure 4.21) calls an internal subroutine Gl to translate an ASCII
character to a hexadecimal value. Gl uses the fact that ASCII letters "0" to "9" are
translated into hexadecimal numbers by subtracting the character value of "0" from them,
and the remaining ASCII characters "A" to "F" are translated into hexadecimal by further
subtracting 7 from the result (because there are seven letters between "9" and "A" in the
ASCII code). The first value obtained from the first letter is shifted to the high nibble
and pushed on the stack. When the second value is obtained from the second letter, it is
combined with the value pulled from the stack.

************************ *
* Find Label
* entry: label character in B, OP code byte (from GETOPCD) in A
* exit: ORs symbol's value into A
* x is unchanged
*
FINDLBL: LDY #LABELS ; y-> first symbol table row
F1: CMPB 2, y+ ; compare first character in B and move to next symbol table entry
BNE Fl ; if mismatch, try next by going to Fl
ORAA -1, y ; OR previous row's value into the OP code in A
RTS ; return to caller
Figure 4.20. Subroutine to Insert a Label as an Operand