PROGRAMMING

5.5.1Addressing Modes for Control Instructions

Table 5-8 lists the addressing modes for the control instructions.

•Relative addressing: The control instruction provides the target address as an 8-bit signed offset (rel) from the address of the next instruction.

•Direct addressing: The control instruction provides a target address, which can have 11 bits (addr11), 16 bits (addr16), or 24 bits (addr24). The target address is written to the PC.

—addr11: Only the lower 11 bits of the PC are changed; i.e., the target address must be in the current 2-Kbyte block (the 2-Kbyte block that includes the first byte of the next instruction).

—addr16: Only the lower 16 bits of the PC are changed; i.e., the target address must be in the current 64-Kbyte region (the 64-Kbyte region that includes the first byte of the next instruction).

—addr24: The target address can be anywhere in the 16-Mbyte address space.

•Indirect addressing: There are two types of indirect addressing for control instructions:

—For the instructions LCALL @WRj and LJMP @WRj, the target address is in the current 64-Kbyte region. The 16-bit address in WRj is placed in the lower 16 bits of the PC. The upper eight bits of the PC remain unchanged from the address of the next instruction.

—For the instruction JMP @A+DPTR, the sum of the accumulator and DPTR is placed in the lower 16 bits of the PC, and the upper eight bits of the PC are FF:, which restricts the target address to the code memory space of the MCS 51 architecture.

Table 5-8. Addressing Modes for Control Instructions

Description

Address Bits

Address Range

Provided

 

 

 

 

 

Relative, 8-bit relative address (rel)

8

-128 to +127 from first byte of next instruction

 

 

 

Direct, 11-bit target address (addr11)

11

Current 2 Kbytes

 

 

 

Direct, 16-bit target address (addr16)

16

Current 64 Kbytes

 

 

 

Direct, 24-bit target address (addr24)†

24

00:0000H–FF:FFFFH

Indirect (@WRj)†

16

Current 64 Kbytes

Indirect (@A+DPTR)

16

64-Kbyte region specified by DPXL (reset

value = 01H)

 

 

 

 

 

†These modes are not used by instructions in the MCS® 51 architecture.

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Intel 8XC251SA, 8XC251SP, 8XC251SQ manual Addressing Modes for Control Instructions, Description Address Bits Address Range
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Embedded Microcontroller, 8XC251SP, 8XC251SA, 8XC251SQ, 8XC251SB specifications

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