Motorola HC12 User Manual
User’s Manual
MCUez HC12 Assembler
144
Assembler Syntax
MOTOROLA
Assembler Syntax
7.4.3.16 Indexed PC versus Indexed PC Relative Addressing Mode
When using the indexed addressing mode with PC as the base register, the
macro assembler allows use of either indexed PC (<offset>, PC) or indexed PC
relative (<offset>, PCR) notation.
macro assembler allows use of either indexed PC (<offset>, PC) or indexed PC
relative (<offset>, PCR) notation.
When indexed PC notation is used, the offset specified is inserted directly in the
opcode.
opcode.
Example:
main:
LDAB 3, PC
DC.B $20, $30, $40, $50
LDAB 3, PC
DC.B $20, $30, $40, $50
In the previous example, register B is loaded with the value stored at address
PC + 3 ($50).
PC + 3 ($50).
When Indexed PC relative notation is used, the offset between the current
location counter and the specified expression is computed and inserted in the
opcode.
location counter and the specified expression is computed and inserted in the
opcode.
Example:
main:
LDAB x4, PCR
x1: DC.B $20
x2: DC.B $30
x3: DC.B $40
x4: DC.B $50
LDAB x4, PCR
x1: DC.B $20
x2: DC.B $30
x3: DC.B $40
x4: DC.B $50
In the previous example, register B is loaded with the value stored at label
x4
($50). The macro assembler evaluates the offset between the current location
counter and the symbol
counter and the symbol
x4
to determine the value, which must be stored in the
opcode.
Inside an absolute section, expressions specified in an indexed PC relative
addressing mode may be:
addressing mode may be:
•
A label defined in any absolute section
•
A label defined in any relocatable section
•
An external label, defined in an XREF directive
•
An absolute EQU or SET label