Types of Registers

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 :

• 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.