UPSD325X datasheet(21/176 Pages) STMICROELECTRONICS

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µPSD325X DEVICES

(3) Register addressing. The

banks,

containing registers R0 through R7, can be ac-

cessed by certain instructions which carry a 3-bit

struction. Instructions that access the registers

this way are code efficient, since this mode elimi-

nates an address byte. When the instruction is ex-

ecuted, one of four banks is selected at execution

time by the two bank select bits in the PSW.

Example:

mov PSW, #0001000B ; select Bank0

mov A, #30H

mov R1, A

(4) Register-specific addressing. Some

in-

structions are specific to a certain register. For ex-

ample, some instructions always operate on the

Accumulator, or Data Pointer, etc., so no address

byte is needed to point it. The opcode itself does

that.

(5) Immediate constants addressing. The val-

ue of a constant can follow the opcode in Program

memory.

Example:

mov A, #10H.

(6) Indexed addressing. Only Program memory

can be accessed with indexed addressing, and it

can only be read. This addressing mode is intend-

ed for reading look-up tables in Program memory.

A 16-bit base register (either DPTR or PC) points

to the base of the table, and the Accumulator is set

up with the table entry number. The address of the

table entry in Program memory is formed by add-

ing the Accumulator data to the base pointer.

Example:

movc A, @A+DPTR

Figure 13. Indexed Addressing

Arithmetic Instructions

The arithmetic instructions is listed in Table 4,

page 22. The table indicates the addressing

modes that can be used with each instruction to

access the <byte> operand. For example, the

ADD A, <byte> instruction can be written as:

ADD a, 7FH (direct addressing)

ADD A, @R0 (indirect addressing)

ADD a, R7 (register addressing)

ADD A, #127 (immediate constant)

Note: Any byte in the internal Data Memory space

can be incremented without going through the Ac-

cumulator.

One of the INC instructions operates on the 16-bit

Data Pointer. The Data Pointer is used to generate

16-bit addresses for external memory, so being

able to increment it in one 16-bit operations is

a useful feature.

The MUL AB instruction multiplies the Accumula-

tor by the data in the B register and puts the 16-bit

product into the concatenated B and Accumulator

registers.

The DIV AB instruction divides the Accumulator by

the data in the B register and leaves the 8-bit quo-

tient in the Accumulator, and the 8-bit remainder in

the B register.

In shift operations, dividing a number by 2n shifts

its “n” bits to the right. Using DIV AB to perform the

division completes the shift in 4?s and leaves the

B register holding the bits that were shifted out.

The DAA instruction is for BCD arithmetic opera-

tions. In BCD arithmetic, ADD and ADDC instruc-

tions should always be followed by a DAA

operation, to ensure that the result is also in BCD.

Note: DAA will not convert a binary number to

BCD. The DAA operation produces a meaningful

result only as the second step in the addition of

two BCD bytes.

AI06643

3Eh

Program Memory

ACC

DPTR

3Ah

1E73h

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