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Instruction Set Summary
2.3.4 Accumulator
ACC is the accumulator. This is the source and destination of most arithmetic instructions, data transfers,
and other instructions. The mnemonic for the accumulator (in instructions involving the accumulator) is A
instead of ACC.
ACC (0xE0) – Accumulator
Bit Name Reset R/W Description
7:0
ACC[7:0]
0x00 R/W Accumulator
2.3.5 B Register
The B register is used as the second 8-bit argument during execution of multiply and divide instructions.
When not used for these purposes, it may be used as a scratchpad register to hold temporary data.
B (0xF0) – B Register
Bit Name Reset R/W Description
7:0
B[7:0]
0x00 R/W B register. Used in MUL/DIV instructions
2.3.6 Stack Pointer
The stack resides in DATA memory space and grows upwards. The PUSH instruction first increments the
stack pointer (SP) and then copies the byte into the stack. The SP is initialized to 0x07 after a reset, and it
is incremented once to start from location 0x08, which is the first register (R0) of the second register bank.
Thus, in order to use more than one register bank, the SP should be initialized to a different location not
used for data storage.
SP (0x81) – Stack Pointer
Bit Name Reset R/W Description
7:0
SP[7:0]
0x07 R/W Stack pointer
2.4 Instruction Set Summary
The 8051 instruction set is summarized in Table 2-3. All mnemonics copyrighted © Intel Corporation,
1980.
The following conventions are used in the instruction set summary:
• Rn – Register R7–R0 of the currently selected register bank
• Direct – 8-bit internal data-location address. This can be DATA area (0x00–0x7F) or SFR area
(0x80–0xFF).
• @Ri – 8-bit internal data location, DATA area (0x00–0xFF) addressed indirectly through register R1 or
R0
• #data – 8-bit constant included in instruction
• #data16 – 16-bit constant included in instruction
• addr16 – 16-bit destination address. Used by LCALL and LJMP. A branch can be anywhere within the
64-KB CODE memory space.
• addr11 – 11-bit destination address. Used by ACALL and AJMP. The branch is within the same 2-KB
page of program memory as the first byte of the following instruction.
• rel – Signed (2s-complement) 8-bit offset byte. Used by SJMP and all conditional jumps. Range is –128
to 127 bytes relative to first byte of the following instruction.
• bit – Direct addressed bit in DATA area or SFR
39
SWRU191C–April 2009–Revised January 2012 8051 CPU
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