Types of Registers

Sun Mar 31, 2013 4:02 pm

WREG Register:
It also named as "Program memory space".

has a size 8-bits.
A temp storage for information at CPU.
WREG stands for Working Register
Can be used for arithmetic and logical instruction like move and add.

Example :

asm code

MOVLW	N

means ; move literal value (N) into WREG register
Note that :

Literal means actual value, certain value not pointer.
N ranges from 0-255 in decimal and from 00-FF in hexadecimal.
Code:
MOVLW 30H or MOVLW 0x30

This says move number 30 in hexadecimal to WREG register.
To write in hexadecimal, number followed by H, or begin the number by 0x.
To write in Binary, B ’the number in binary’.
To write in decimal, “.” Then the number in decimal.

Example :
Add two numbers 50H + 1FH in WREG register:
asm code

MOVLW 	50H
ADDLW	1FH

File Register:
It also called Data Memory Space, they exist in RAM
Consists of:

Special Function Registers (SFR):
Which are dedicated to special functions such as ALU status, timers, serial communication and I/O ports. Examples: (PORTA, PORTB, PORTC, LATA, LATB, LATC, TRISA, TRISB, TRISC). They are fixed numbers per microprocessor. Their counts range from 7 bytes up to 158 bytes. Also the size of each one of them is 8-bits(1 Byte).
General Purpose Registers: (GP-RAM):
Used for data storage in RAM, spaces that not allocated to SFR is used for GP-RAM. The size of each register is 8-bits( 1 Byte).

It is also important to talk about "Access Bank:"
File Register is divided into 256-byte blocks or banks. This bank called access- bank. At least one access block exists in any microprocessor. For PIC18 family, these 256-bytes are divided into 128 bytes for GP-RAM and 128 bytes for SFRs.

Status registers:
It is a 8bit register , each bit is used for specific status.

C: Carry Flag: is set to 1 whenever there is a carry out from the D7 bit.
DC: Digital Carry Flag: is set to 1 whenever there is a carry out from D3 to D4.
Z: Zero flag: is set to 1 if the arithmetic or logical operation result is zero, and zero otherwise.
OV: Overflow flag: is set to 1 whenever the result of a signed number operation is too large, causing high order bit to overflow into sign bit.
N: Negative flag: N=0, the result is positive, else if N=1, the result is negative.

Example:
asm code

MOVLW 38H
ADDLW 2FH

Answer :

Code:: 38H -> 0011 1000 + 2FH-> 0010 1111 --------------------------------------- 67H 0110 0111

C=0 , there is no carry after D7.
DC=1 , there is a carry from D3 to D4.
Z=0; the value at WREG is not zero.