Realization of high performance run time loadable mips soft-core processor

IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 | pISSN: 2321-7308
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Volume: 03 Issue: 04 | Apr-2014, Available @ http://www.ijret.org 252
REALIZATION OF HIGH PERFORMANCE RUN-TIME LOADABLE
MIPS SOFT-CORE PROCESSOR
Prathibha S R1
, M Z Kurian2
1
M.Tech Student, Department of E&C, Sri Siddhartha Institute of Technology, Tumkur, Karnataka, India
2
Head, Department of E&C, Sri Siddhartha Institute of Technology, Tumkur, Karnataka, India
Abstract
Soft-core processors on Field Programmable Gate Array (FPGA) chips are becoming a solution for application-specific
customization. Soft-core processors provide several advantages for designer. Currently using microprocessor doesn’t support real-
time loading. If, any change in the assembly code of the implemented processor, it requires re-implementation and downloads the soft-
core on FPGA. This project proposes realization of run-time loading technique of a MIPS (Microprocessor without Interlocked
Pipeline Stages) soft- core processor on FPGA. The proposed system consists of mainly three components: microprocessor soft-core,
software tool and universal asynchronous Receiver/Transmitter (UART). Since, here MIPS code is updating instantly there is no need
of resynthesize, place, route, and reload the soft-core. The software tool communicates between the user and MIPS soft-core
processor through UART. Five stage pipelining is included to improve the overall processor performance. In this paper 32-bit MIPS
soft-core processor, RAM module, APB Interface is designed and simulated using Modelsim. Design architecture will be doing in
Verilog, simulate using Modelsim 10.3 simulator and realize in Spartan-6 FPGA using Xilinx ISE 14.2.
Keywords: FPGA, MIPS, PERL, RISC, UART etc…
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1. INTRODUCTION
Soft-core processor is a microprocessor their architecture and
behavior are defined in Hardware Description Language
(HDL), which can be synthesized in programmable hardware,
like FPGAs or ASICs. MIPS is a RISC processor, which is a
32-bit processor designed with single cycle
implementation.[2] For the realization of MIPS soft-core
processor using Field Programmable Gate Array (FPGA)
many techniques have been reported and documented in open
literature. However, the run time loading for CPU on FPGA
and its hardware realization using FPGA has not been reported
as of our knowledge.
This project is to propose run-time loading technique of
machine code for MIPS-32 soft-core processor on FPGA.
Some logic has to be introduced in the program memory made
it rewritable in runtime. ROM (Read Only Memory) logic has
to be changed to RAM. So bipartite the RAM (Random
Access Memory) into two sections, one section is for
instruction memory and the other one is used as data
memory.[9] The software tool is build by using Perl
programming language. It is used to write the MIPS assembly
code, debug the code, generates the machine code and sends it
to the FPGA.
The objective of this project is to design and implementation
of run-time loading of machine code for MIPS-32 soft-core
processor on FPGA. Write the PERL (Practical Extract and
Report Language) script to generate the opcode of the MIPS
processor and that should be stored in text file and after
storing the opcode information that sent to the instruction
decoder through UART/RS232 bus protocols and that will
fetch the correct opcode from that instruction decoder and
send to the MIPS logic through Advance Peripheral Bus
protocol (APB) interface and it can change in the run time
through programming the FPGA. Implementing the required
MIPS using FPGA that controls the processor at run time and
it debug easily which gives the complete functional design of
the MIPS processor. Five stage pipelining is included to
improve the overall processor performance. [1]
The organization of this paper is as follows. Section II
presents proposed work, Section III discussed on system
design of proposed work, Section IV shows the results.
Section V concludes this work and future enhancement.
2. PROPOSED WORK
The fig.1 shows the block diagram of the proposed system.
Writing the perl script to generates the opcode and operand
code fields in a text file format.
Text file: It stores the opcode and operand information in
machine level language.
UART: From computer side it is the media to send and
receive the data serially to/from external world. The UART is
used to connect the FPGA and computer for data and

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instruction transfer. This UART information will be stored on
the FPGA RAM for further processing.
MIPS Logic: The architecture is standard processor
architecture with fetch, decode and execute blocks with run
time instruction execution. The internal blocks of MIPS logic
are data and instruction buffers, ALU controller and ALU. The
data and instructions will be feed from RAM.
Fig -1: Block Diagram of a system
APB: This is one of the standard bus protocol defined by
ARM Company which is used to interconnect the modules.
3. SYSTEM DESIGN
3.1 Design of MIPS Processor
MIPS processor is 32-bit processor designed with single cycle
implementation operate with frequency of 100MHz.
Fig -2: Architecture of proposed MIPS Processor
As shown in fig.2 the numerical 1, 2, 3 indicate as below;
1. Waiting_For_Decoder: Wait for the fetched
instruction if it is not fetched any instruction in
decoder module.
2. Waiting_For_Execution: After completion of
decoding it will wait for next execution done.
3. Decoder_Fetch: Fetch the valid instruction from the
fetch module through advance peripheral bus.
The Fig.2 shows the architecture of the proposed processor. It
mainly consists of Fetch module, Decode module, execution
module, register bank and updated PC.
Fetch module: In fetch unit instruction is fetched from
memory. The address of the instruction will be fetched from
program counter. The fetched instruction is again moved to
the instruction register through data bus.
Decode module: Here transfer the instruction from instruction
register to decode unit. In this unit the fetched instruction is
decoded by the processor. It will get the operands and opcode
given in the instruction.
Execute module: In this unit executes the instruction. ALU
performs arithmetic and logical operation depending on
requirement of the instruction and result is stored back in
register/memory.
Register Bank: It contains array of registers. It has 32 registers
each of 32bit wide.
PC Update: Update the value of program counter normally
incremented by 1. If any jump or branch instructions PC
updated to other address.
Interrupt Control: It allows interrupts on one or more CPU
lines, based on priority assigned. When interrupt occurs,
disturbs the execution of current code in processor and needs
immediate attention.
Flush Signal: As soon as flush signal received, it cancels
instruction queues cannot performing any other operations.
Program counter jumps to specific address, start fetching in
normal way.
Control Signals are
1. Fetch instruction: This signal from fetch unit to RAM
2. Valid fetch: This signal from RAM to fetch unit valid
instruction is fetched.
3. Data memory request: This signal from execution unit
to the Data memory requesting for access memory.
4. Valid read data: This signal from memory to the
execution unit after read the data.
5. Interrupt: This signal from external device to the
interrupt control.

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6. Data memory operation: This signal form data
memory to the execution unit.
7. Reset: Global signal.
8. Clock: Global signal.
3.1.1 Design of Fetch Unit
Fig -3: Fetch unit of MIPS Processor
Above fig.3 shows Fetch unit of the MIPS processor.
Simulation result of the Fetch unit is shown in fig 8.
Clk, Reset: Global signals.
Valid fetch: It will fetch the valid instruction from the ram
module through APB.
Decoder fetch: Decoder sends this signal to fetch unit to send
next fetched instruction.
Flush: It will flush the entire module when jump or call
instruction comes.
Fetched Instruction: It will fetch the instruction (data and
address) from the RAM.
Updated PC: It is used to update the instruction address in the
fetch module.
Fetch Instruction (IF): The instruction is fetched from
memory and placed in the instruction register (IR).
Waiting for decoder: Fetched instruction is waiting for
decoder to decode the operands and opcode.
Program counter: It is a register that contains the address
(location) of the instruction being executed at the current time.
After each instruction gets fetched, the program counter
increases its value by 1 and points to the next instruction in the
execution sequence. When the processor restarts or reset, the
program counter normally back to 0.
3.1.2 Design of Decode Unit
Fig -4: Decode unit of the MIPS Processor
Above fig.4 shows Decode unit of the MIPS processor.
Simulation result of the Decode unit is shown in fig 9.
Waiting for decoder: when this signal is high, indicates that
the fetch module is waiting for decoder to decode next
instruction.
Flush: It will flush the entire module when jump or call
instruction.
Instruction for decoder: Fetch unit sends 32-bit instruction to
the decoder module.
Execution done: After execution completes, execution module
sends this signal to the decoder unit.
Decoder fetch: Decoder sends this signal to fetch unit to send
next fetched instruction.
Waiting for execution: After completion of decoding it will
wait for next execution.
Control signals: which control all instruction like opcode,
fetch, program counter etc,.
3.1.3 Design of Execution Unit
Fig -5: Execution unit of MIPS Processor
The fig.5 shows Execution unit of the MIPS processor. The
fig.10 shows the simulation result of Execution unit.

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Waiting for execution: After completion of decoding it will
wait for next execution.
Control signals: It controls all instruction like opcode, fetch,
program counter etc.
Memory operation done: After complete execution from the
execution unit it will send the acknowledgement to memory
that it completes operation.
Execution done: when it will high indicates that the execution
of the instruction is complete.
Data memory request: Requesting for the data memory to
store the data.
Data memory operation: This stores required data in the data
memory.
Data memory address: This stores required address in the data
memory.
3.2 RAM Module
The soft-core processor has RAM and it is 32 bit aligned.
Some of the internal signals of RAM are Address, Data_out,
Read, Ready, Clock, Reset.
Fig -6: State Diagram of RAM
Above fig.6 shows state diagram of RAM. To support real
time loading and ROM logic has to be changed to RAM.
ROM (Read Only Memory) is called instruction memory can’t
be rewritable. Program memory made it rewritable in runtime,
so bipartite the RAM (Random Access Memory) into nearly
two equal sections. One section is for instruction memory and
the other one is used as data memory approximately 2KB
each.
When WE (Write enable) =1, Write the data in to RAM.
When WE (Write enable) =0, Read the data from the RAM.
The fig.12 shows the simulation result of RAM unit.
3.3 Advance Peripheral Bus Protocol
APB is a part of the AMBA 3 (Advanced Microcontroller Bus
Architecture) protocol family. It gives a low-cost interface that
is optimized for minimal power consumption and reduced
interface complexity. APB interfaces to any peripherals
having low-bandwidth and do not require the pipelined bus
interface. It has non-pipelined bus and synchronous. All signal
transitions are only related to the rising edge of the clock to
enable the integration of APB peripherals. Each transfer takes
minimum two cycles. It is used for write and read transfers.
The below tabel.1shows the list of APB signals with its
description.
Table -1: APB Signal Descriptions
Sl
No
Signals Description
1 PREADY
A ready signal used by slaves to
extend a APB transfer
2 PSLVERR
An error signal indicate the failure of
a transfer (Valid only
PSEL,PENABLE,PREADY are
high)
3 PCLK
The rising edge of PCLK times all
transfers on APB
4 PADDR
APB address bus can be extend up to
32 bits and driven by peripheral bus
bridge
5 PWRITE
This signal indicates an APB write
access when HIGH and APB read
access when LOW
6 PSEL
APB bridge unit generates this signal
to each slave devise and select the
desired one and transfer the data
7
PENABL
E
Enable signal indicated the second
and further cycles used for an APB
transfer
8 PWDATA
The write data, it is 32 bits wide and
operates when PWRITE = 1
9 PRDATA
The read data, it is 32 bits wide and
operates when PWRITE = 0
10 PRESET
APB reset signal is active LOW, its
normally connected to the system bus
reset signal
Operating States
The operation of APB is as shown in fig.7. The state machine
consists of three states:

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Fig -7: State diagram of operating states of APB
IDLE: It is a default state of APB.
SETUP: When a transfer is required this bus moves to the
SETUP state. An appropriate select signal PSEL is chosen
and it remains in this state for one clock cycle and next jump
to ACCESS state on next rising edge of the clock.
ACCESS: When PENABLE is HIGH, ACCESS state is
activated. The address, write, select, and write data signals
stable during transition from SETUP to ACCESS state. This
state is controlled by the PREADY signal from the slave:
PREADY =0, Bus will remains in the ACCESS state.
PREADY=1, Bus returns to IDLE state, if no other transfers
are required and Bus directly moves to SETUP state still
transfers are there.
The fig.13 shows the simulation result of APB.
4. RESULTS
Fig -8: Simulation Result of Fetch Unit
Fig -9: Simulation Result of Decode Unit
Fig -10: Simulation Result of Execution Unit
Fig -11: Top Level RTL Schematic of MIPS Processor

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Fig -12: Simulation Result of RAM
Fig -13: Simulation Result of APB
5. CONCLUSIONS AND FUTURE WORK
This paper proposes design and simulation result of fetch unit,
decode unit, execution unit, MIPS processor and RAM
module of 32-bit Run-Time Loadable MIPS soft-core
processor and also includes APB Interface Block. The
proposed system is trying to utilize less FPGA resources and
reduce area consumption to consume small area out of the
entire FPGA leaving plenty of FPGA resources for
implementing other parallel processors on the same device. It
focus on remaining blocks of system design, different
optimization techniques such as pipelining will be applied in
order to obtain high performance considering the parameters
like speed and area.
REFERENCES
[1] Balaji valli, A. Uday Kumar, B.Vijay Bhaskar, “FPGA
Implementation and Functional Verification of a
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Computational Engineering Research (ijceronline.com)
Vol. 2 Issue. 5, September-2012.
[2] Jason G. Tong, Ian D. L. Anderson and Mohammed A.
S. Khalid, “Soft-Core Processors for Embedded
Systems” 18th International Confernece on
Microelectronics (ICM) 2006
[3] Ortega-Sanchez C., " MiniMIPS: An 8-Bit MIPS in an
FPGA for Educational Purposes," in Proc. of
International Conference on Reconfigurable
Computing and FPGAs,pp. 152-157, 2011.
[4] Wael M ElMedany, Khalid A AlKooheji “Design and
Implementation of a 32bit RISC Processor on Xilinx
FPGA”.
[5] V.N.Sireesha, D.Hari Hara Santosh, “ FPGA
Implementation of A MIPS RISC Processor”
Int.J.Computer Technology & Applications,Vol 3 (3),
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[6] Galani Tina, R.D.Daruwala “Performance
Improvement of MIPS Architecture by Adding New
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in Computer Science and Software Engineering 3(2),
February - 2013, pp. 1-6.
[7] Prashant D Bhirange, V. G. Nasre, M. A. Gaikwad3,
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[8] Rupali S. Balpande, Rashmi S. Keote “Design of
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[9] Mazen Bahaidarah, Hesham Al-Obaisi “A Novel
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