Atmel C51 Manual descarregar pdf (Página 10)

The 8051 Instruction Set

Atmel 8051 Microcontrollers Hardware Manual 1-8

4316E–8051–01/07

To right-shift by an odd number of digits, a one-digit shift must be executed. Table 1-6

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 Accumu-

lator are shown alongside each instruction.

First, pointers R1 and R0 are set up to point to the two bytes containing the last four

BCD digits. Then a loop is executed which leaves the last byte, location 2EH, holding

the last two digits of the shifted number. The pointers are decremented, and the loop is

Table 1-5. Shifting a BCD Number Two Digits to the Right

2A 2B 2C 2D 2E ACC

MOV A,2EH

MOV 2EH, 2DH

MOV 2DH, 2CH

MOV 2CH, 2BH

MOV 2BH, # 0

Note: Using direct MOVs: 14 bytes, 9 µs

2A 2B 2C 2E 2E ACC

CLR A

XCH A,2BH

XCH A,2CH

XCH A,2DH

XCH A,2EH

Note: Using XCHs: 9 bytes, 5 µs

Table 1-6. Shifting a BCD Number One Digit to the Right

2A 2B 2C 2D 2E ACC

MOV R1,# 2EH 00 12 34 56 78 XX

MOV R0, # 2DH 00 12 34 56 78 XX

loop for R1 = 2EH:

LOOP: MOV A, @R1 00 12 34 56 78 78

XCHD A, @R0 00 12 34 58 78 76

SWAP A 00 12 34 58 78 67

MOV @R1, A 00 12 34 58 67 67

DEC R1 00 12 34 58 67 67

DEC R0 00 12 34 58 67 67

CJNE R1, #2AH, LOOP

loop for R1 = 2DH: 00 12 38 45 67 45

loop for R1 = 2CH: 00 18 23 45 67 23

loop for R1 = 2BH: 08 01 23 45 67 01

CLR A 08 01 23 45 67 00

XCH A,2AH 00 01 23 45 67 08

1 2 ... 5 6 7 8 9 10 11 12 13 14 15 ... 115 116

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Atmel C51 Manual Página 10