Register Transfer Language (RTL)

In symbolic notation, it is used to describe the micro-operations transfer among registers. It is a kind of intermediate representation (IR) that is very close to assembly language, such as that which is used in a compiler.The term “Register Transfer” can perform micro-operations and transfer the result of operation to the same or other register. 

Micro-operations :
The operation executed on the data store in registers are called micro-operations. They are detailed low-level instructions used in some designs to implement complex machine instructions. 

Register Transfer :
The information transformed from one register to another register is represented in symbolic form by replacement operator is called Register Transfer. 

Replacement Operator :
In the statement, R2 <- R1, <- acts as a replacement operator. This statement defines the transfer of content of register R1 into register R2. 

There are various methods of RTL – 

1. General way of representing a register is by the name of the register enclosed in a rectangular box as shown in (a). 
   
2. Register is numbered in a sequence of 0 to (n-1) as shown in (b). 
   
3. The numbering of bits in a register can be marked on the top of the box as shown in (c). 
   
4. A 16-bit register PC is divided into 2 parts- Bits (0 to 7) are assigned with lower byte of 16-bit address and bits (8 to 15) are assigned with higher bytes of 16-bit address as shown in (d). 
   

Basic symbols of RTL :

Symbol	Description	Example
Letters and Numbers	Denotes a Register	MAR, R1, R2
( )	Denotes a part of register	R1(8-bit) R1(0-7)
<-	Denotes a transfer of information	R2 <- R1
,	Specify two micro-operations of Register Transfer	R1 <- R2 R2 <- R1
:	Denotes conditional operations	P : R2 <- R1 if P=1
Naming Operator (:=)	Denotes another name for an already existing register/alias	Ra := R1

Register Transfer Operations:

The operation performed on the data stored in the registers are referred to as register transfer operations.

There are different types of register transfer operations:

1. Simple Transfer – R2 <- R1

The content of R1 are copied into R2 without affecting the content of R1. It is an unconditional type of transfer operation. 

2. Conditional Transfer – 

It indicates that if P=1, then the content of R1 is transferred to R2. It is a unidirectional operation. 

3. Simultaneous Operations –
If 2 or more operations are to occur simultaneously then they are separated with comma (,). 

If the control function P=1, then load the content of R1 into R2 and at the same clock load the content of R2 into R1.