Computer Organization and Architecture

Register Transfer Language

• A register transfer language is a system for expressing microoperation sequences among registers in symbolic form.
• It is a convenient tool for describing the internal organization of digital computers concisely and precisely.

Registers

• Computer registers are designated by upper case letters (and optionally followed by digits or letters) to denote the function of the register.
• Examples of register designations include MAR (memory address register), PC (program counter), IR (instruction register), and R1 (processor register).
• The individual flip-flops in an n-bit register are numbered in sequence from 0 through n-1, starting from 0 in the rightmost position and increasing the numbers toward the left.

Representation of Registers

• A register is commonly represented by a rectangular box with the name of the register inside.
• Individual bits can be distinguished, and the numbering of bits in a register can be marked on top of the box.
• A 16-bit register can be partitioned into two parts: the low-order byte (bits 0 through 7) and the high-order byte (bits 8 through 15).

Register Transfer

• Information transfer from one register to another is designated in symbolic form by means of a replacement operator.
• The statement R2← R1 denotes a transfer of the content of register R1 into register R2.

Basic Definitions

• A digital system is an interconnection of digital hardware modules, including registers, decoders, arithmetic elements, and control logic.
• Digital modules are best defined by the registers they contain and the operations performed on the data stored in them.
• A microoperation is an elementary operation performed on the information stored in one or more registers.

Microoperations

• Examples of microoperations include shift, count, clear, and load.
• The internal hardware organization of a digital computer is best defined by specifying the set of registers, the sequence of microoperations, and the control that initiates them.

Register Transfer Language (RTL)

• RTL is a symbolic notation used to describe the micro-operation transfer among registers.
• It provides an organized and concise manner for listing the micro-operation sequences in registers and the control functions that initiate them.
• The RTL statement R2← R1 designates a replacement of the content of R2 by the content of R1.
• The content of the source register R1 does not change after the transfer.

Control Function

• A control function is a Boolean variable that is equal to 0 or 1.
• The control function is included in the statement as P: R2← R1, where P is the control signal generated by a control section.
• The control condition is terminated by a colon, implying that the transfer operation is executed by the hardware only if P=1.