UPSD3233 datasheet(20/170 Pages) STMICROELECTRONICS

20 / 170 page

uPSD3234A, uPSD3234BV, uPSD3233B, uPSD3233BV

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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 uPSD323X 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., page 21 shows how this can be

done using XCH instructions. To aid in under-

standing 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 in-

struction 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., page 21 shows a sample of code that will right-

shift a BCD number one digit, using the XCHD in-

struction. Again, the contents of the registers hold-

ing the number and of the accumulator are shown

alongside 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>

XXXX

MOV <dest>,A

<dest> = A

MOV <dest>,<src>

XXXX

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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