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Special Assembler Symbols

The Ax51 assembler defines and reserves names of the x51 register set. These predefined names are used in x51 programs to access the processor registers. Following, is a list of the each of the 8051, 80C51MX, and 251 registers along with a brief description:

Register Description
A Represents the 8051 Accumulator. It is used with many operations including multiplication and division, moving data to and from external memory, Boolean operations, etc.
DPTR The DPTR register is a 16 bit data pointer used to address data in XDATA or CODE memory.
PC The PC register is the 16 bit program counter. It contains the address of the next instruction to be executed.
C The Carry flag; indicates the status of operations that generate a carry bit. It is also used by operations that require a borrow bit.
AB The A and B register pair used in MUL and DIV instructions.
R0 – R7 The eight 8 bit general purpose 8051 registers in the currently active register bank. A Maximum of four register banks are available.
AR0 – AR7 Represent the absolute data addresses of R0 through R7 in the current register bank. The absolute address for these registers changes depending on the register bank that is currently selected. These symbols are only available when the USING assembler statement is given. Refer to the USING assembler statement for more information on selecting the register bank. These representations are suppressed by the NOAREGS directive off the Cx51 compiler.
PR0, PR1 The Universal Pointer Registers of the 80C51MX architecture. Universal Pointer can access the complete 16MB address space of the 80C51MX. PR0 is composed of registers R1, R2, and R3. PR1 is composed of registers R5, R6, and R7.
EPTR Additional extended data pointer register of the 80C51MX architecture. EPTR may be used to access the complete memory space.
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