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CMX850L8 View Datasheet(PDF) - CML Microsystems Plc

Part Name
Description
MFG CO.
CMX850L8
CML
CML Microsystems Plc CML
'CMX850L8' PDF : 103 Pages View PDF
Communications Controller
CMX850
1.5.14 8051 Accumulator, Flags and Stack Pointer
1.5.14.1
Accumulator Registers (A,B)
A (ACC): SFR Address $E0
All bits reset to 0 on reset. This register is bit addressable.
Bit:
7
6
5
4
3
2
1
0
Bits 7 – 0 of the A register
The main accumulator register A is used in a large number of register-specific arithmetic, logical
and data transfer instructions. No address byte is needed to point to it, the opcode itself does that.
Instructions that refer to the main accumulator as A assemble as accumulator-specific instructions.
The A register is also mapped into the 8051 SFR space, and can be read or written using direct
addressing. Instructions that refer to the accumulator as ACC will access the accumulator using
direct addressing. For example, the instructions MOV R0,A and MOV R0,ACC will both copy the
contents of the accumulator to register R0, but will assemble as different opcodes – in this example
the main difference is that the instruction MOV R0,A assembles as a 1 byte opcode rather than 2,
and executes in 1 machine cycle rather than 2.
B: SFR Address $F0
All bits reset to 0 on reset. This register is bit addressable.
Bit:
7
6
5
4
3
2
1
0
Bits 7 – 0 of the B register
Register B is mapped into the 8051 SFR space, but is also used implicitly in the MUL AB and DIV
AB instructions. The B register is commonly used by programmers as an auxiliary register to store
temporary data.
1.5.14.2
Program Status Word Register (PSW)
PSW: SFR Address $D0
All bits reset to 0 on reset. This register is bit addressable.
Bit:
7
6
5
4
3
2
1
0
Carry
flag
(CY)
Auxiliary
Carry
flag
(AC)
General
purpose
flag 0
(F0)
Register
bank
select 1
(RS1)
Register
bank
select 0
(RS0)
Overflow
flag
(OV)
General
purpose
flag 1
(F1)
Parity
flag
(P)
PSW Register b7: Carry flag (CY)
During ADD, ADDC or SUBB instructions, this bit gets set if there is a carry-out from bit 7, and
cleared otherwise. The Carry flag is also affected by certain other arithmetic, logical and program
branching instructions, and also acts as a single bit “accumulator” for a number of the Boolean
manipulation instructions.
© 2003 CML Microsystems Plc
58
D/850/6
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