23-03-2012, 02:50 PM
8 BIT RISC MICROPROCESSOR IMPLEMENTED ON FPGA Using HDL
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INTRODUCTION :
Microprocessors and Microcontrollers have traditionally been designed around two
philosophies: Complex Instruction Set Computer (CISC) and Reduced Instruction Set
Computer (RISC). The CISC concept is an approach to the Instruction Set Architecture
(ISA) design that emphasizes doing more with each Instruction using a wide variety of
addressing modes, variable number of operands in various locations in its Instruction Set.
As a result, the Instructions are of widely varying lengths and execution times thus
demanding a very complex Control Unit, which occupies a large real estate on chip.
On the other hand, the RISC Processor works on reduced number of Instructions, fixed
instruction length, more general-purpose registers, load-store architecture and simplified
addressing modes which makes individual instructions execute faster, achieve a net gain
in performance and an overall simpler design with less silicon consumption as compared
to CISC. This gives the RISC Architecture more room to add on-chip peripherals,
interrupt controllers and programmable timers. The above features make the RISC design
ideally suited to participate in a powerful trend in the embedded Processor market – the
"system-on-a-chip".
REGISTER FILE
Arithmetic and Logical instructions require three source registers and one destination
register. Of the three source registers, two are used as input registers and the other is used
as an instruction register. A total of 4 bits would be required for any ALU instruction.
CONTROL UNIT
The Control Unit is the heart of the Microprocessor. It accepts as input, those signals that
are needed to operate the Controller, and provides as output all the control signals
necessary to effect that operation. Figure 2 shows a block level view of the Control Unit,
with its input and output signals. The outputs of the Control Unit are the control signals
that generate the control sequences for the operational codes of the machine.
ARITHMETIC & LOGIC UNIT (ALU)
It has 8-bit inputs and an output . The ALU takes
two operands from the A and B registers. The result register Z is used to hold the ALU
output. The ALU has the capability to perform 9 operations. After
every ALU instruction, the output register is updated.
The various units inside the ALU are described below:
Adder/Subtractor: The 8-bit adder/subtractor in the ALU is a Reduced Full
Adder built by using universal gates. A ripple carry adder is used in which the
carry signal propagates from the LSB to the MSB and coming out as Cout.
Logical Unit: It provides all the possible logical operations (nand, nor, exor,
not,and,or,xnor) in the ALU.
APPLICATIONS :
High speed data transmission,real time applications,control applications etc….
Some of the real time applications are: in all of IBM's supercomputers , In Microsoft's Xbox 360 and Sony's PlayStation 3.
In many embedded applications like printers and cars.
Atmel AVR is a RISC mrocessor which is used in a variety of products ranging from Xbox handheld controllers to BMW cars.