The modules of a digital system are best defined by a set of registers and the operations that are performed on the binary information stored in them. Examples of register operations are shift, count, clear. and load. Registers are assumed to be the basic components of the digital system. The flow and processing performed on the data stored in the registers are referred 10 as register transfer Operations, We'll see subsequently how a hardware description language includes operation; that correspond to the register transfer operations of a digital system.A digital system is represented at the register transfer level (R1l..) when it is specified by the following three components:
1. The set of registers in the system.
2. The operations that are performed on the data stored in the registers.
The control that supervises the sequence of operations in the system. A register is a group of flip-fops that stores binary intonation and has the capability of performing o ne or more elementary operations. A register can load new information or shift the information to the right or the le ft. A counter is. considered a regjster that increments a number by a fixed value (e .g.• I ).A flip-flop is considered a one-bit register that can be set. cleared. or complemented. In fact. the flip -flops and associated gates of any sequential circuit can be called registers by this definition. The operations executed on the information stored in registers are elementary operations that are perfumed in parallel on a data wor consisting of bits during one clock cycle. The data produced by the ope ration may replace the binary information that was in the register before the operation executed. Alternatively. me result may be transferred to another register (i.e.• an operation on a register may leave its contents unchanged). Th e digital circuits introduced in registers that implement elementary operatio ns. A counter with a parallel load is able to perform me incremcnt-by-one and load operations. A bidirectional shift register is able to perform me shift-rig ht and shift-left operations.
The operations in a digital system are controlled by timing signals that sequence the operations in a prescribed manner. Certain conditions that depend on results of previous operations may determine the sequence of future operations. The outputs of the control log ic are binary variables that initiate the various operations in the system's registers
.Information trasfer from one register to another is designated in symbolic form by mean s of a replacement operator. The denotes a transfer of the contents of register RJ into register R2---that is. a replacement of the contents of register R2 by the contents of register RJ. By definition. the contents of the source register RI do not change after the transfer. They are merely copied to RI . symbolizes the transfer and its direction: it points from the register whose con tents are being transferred and towards the register that will receive the contents . A control signal would determine when the operation actually executes.
Rl - Rl + R2 R3-R3 + I R4 - shrR4
R5 -0
Add contents of R2 to Rl (RI gets Rl + R2) Increment R3 by I (count upwards )
Shift right R4 Clear R5
1. Transfer operations. which transfer data from one register to another.
2. Arithmetic operations. which perform arithmetic on data in registers.
3. Logic operations. which perform bit manipulation (e.g., logical OR) of nonnumeric data in registers.
4. Shift operations. which shift data between registers.
The transfer operation does not change the information content of the data being moved from the source register to the destination register. The other three operations change the n content during the transfer. The register transfer notation and the symbols used to represent the various register transfer operations are not standardized. In this text, we employ two types of notation. The notation introduced in this section will be used infonn ally to specify and explain digital systems at the register transfer level. The next section introduces the RTL symbols used in the Verilog HDL.
