UPSD323X datasheet(24/176 Pages) STMICROELECTRONICS

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µPSD323X

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Data Transfers

Internal RAM. Table 6 shows the menu of in-

structions that are available for moving data

around within the internal memory spaces, and the

addressing modes that can be used with each

one. The MOV <dest>, <src> instruction allows

data to be transferred between any two internal

RAM or SFR locations without going through the

Accumulator. Remember, the Upper 128 bytes of

data RAM can be accessed only by indirect ad-

dressing, and SFR space only by direct address-

ing.

Note: In

µPSD323X Devices, the stack resides in

on-chip RAM, and grows upwards. The PUSH in-

struction first increments the Stack Pointer (SP),

then copies the byte into the stack. PUSH and

POP use only direct addressing to identify the byte

being saved or restored, but the stack itself is ac-

cessed by indirect addressing using the SP regis-

ter. This means the stack can go into the Upper

128 bytes of RAM, if they are implemented, but not

into SFR space.

The Data Transfer instructions include a 16-bit

MOV that can be used to initialize the Data Pointer

(DPTR) for look-up tables in Program Memory.

The XCH A, <byte> instruction causes the Accu-

mulator and ad-dressed byte to exchange data.

The XCHD A, @Ri instruction is similar, but only

the low nibbles are involved in the exchange. To

see how XCH and XCHD can be used to facilitate

data manipulations, consider first the problem of

shifting and 8-digit BCD number two digits to the

right. Table 8 shows how this can be done using

XCH instructions. To aid in understanding how the

code works, the contents of the registers that are

holding the BCD number and the content of the

Accumulator are shown alongside each instruction

to indicate their status after the instruction has

been executed.

After the routine has been executed, the Accumu-

lator contains the two digits that were shifted out

on the right. Doing the routine with direct MOVs

uses 14 code bytes. The same operation with

XCHs uses only 9 bytes and executes almost

twice as fast. To right-shift by an odd number of

digits, a one-digit must be executed. Table 9

shows a sample of code that will right-shift a BCD

number one digit, using the XCHD instruction.

Again, the contents of the registers holding the

number and of the accumulator are shown along-

side each instruction.

Table 6. Data Transfer Instructions that Access Internal Data Memory Space

Mnemonic

Operation

Addressing Modes

Dir.

Ind.

Reg.

Imm

MOV A,<src>

A = <src>

MOV <dest>,A

<dest> = A

MOV <dest>,<src>

MOV DPTR,#data16

DPTR = 16-bit immediate constant

PUSH <src>

INC SP; MOV “@SP”,<src>

POP <dest>

MOV <dest>,”@SP”; DEC SP

XCH A,<byte>

Exchange contents of A and <byte>

XCHD A,@Ri

Exchange low nibbles of A and @Ri

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