Processor Interface ASIC (PI ASIC) was widely used to provide processor interface logics for MAR31750 Processor along with MA31751 Memory Management Unit (MMU) chip. However with the diminishing availability of MA31750 processors, use of the Mil-Std -1750 processor of Honeywell make – HX1750 as a replacement for the obsolete MAR 31750 Since HX-1750 processor does not have a compatible COTS MMU chip available, it was decided to functionality of the existing MMU chip (MA31751) onto an FPGA The remaining regular processor interface logics, similar to those in PI ASIC are also housed inside the Processor Interface FPGA. Considering the voluminous size of requirements, RTAX-1000S FPGA was chosen to implement the processor interface logics.

1. http://www.iaeme.com/IJECET/index.asp 46 editor@iaeme.com
International Journal of Electronics and Communication Engineering & Technology
(IJECET)
Volume 7, Issue 3, May–June 2016, pp. 46–52, Article ID: IJECET_07_03_006
Available online at
http://www.iaeme.com/IJECET/issues.asp?JType=IJECET&VType=7&IType=3
Journal Impact Factor (2016): 8.2691 (Calculated by GISI) www.jifactor.com
ISSN Print: 0976-6464 and ISSN Online: 0976-6472
© IAEME Publication
DESIGN AND DEVELOPMENT OF
MEMORY MANAGEMENT UNIT FOR MIL-
STD-1750 PROCESSOR
Prasad. S.G and Dr Siva Yellampalli
VTU Extension Centre, UTL Technologies Limited, Bangalore, Karnataka, India
Naveen. V
Control and Digital Electronics Group, ISAC, Department of Space, Bangalore,
Karnataka, India
ABSTRACT
Processor Interface ASIC (PI ASIC) was widely used to provide processor
interface logics for MAR31750 Processor along with MA31751 Memory
Management Unit (MMU) chip. However with the diminishing availability of
MA31750 processors, use of the Mil-Std -1750 processor of Honeywell make –
HX1750 as a replacement for the obsolete MAR 31750 Since HX-1750
processor does not have a compatible COTS MMU chip available, it was
decided to functionality of the existing MMU chip (MA31751) onto an FPGA
The remaining regular processor interface logics, similar to those in PI ASIC
are also housed inside the Processor Interface FPGA. Considering the
voluminous size of requirements, RTAX-1000S FPGA was chosen to
implement the processor interface logics. With a system clock of 24MHz and
an internal MMU inside the FPGA the cycle time for memory accesses has
considerably been reduced. The embedded SRAM blocks of the RTAX-1000S
FPGA are used as shared RAM between CPU and the 1553 device.
Key words: MMU, FPGA, ASIC, BPU
Cite this Article: Prasad. S. G, Dr Siva Yellampalli and Naveen. V, Design
and Development of Memory Management Unit for MIL-STD-1750
Processor, International Journal of Electronics and Communication
Engineering & Technology, 7(3), 2016, pp. 46–52.
http://www.iaeme.com/IJECET/issues.asp?JType=IJECET&VType=7&IType=3

2. Design and Development of Memory Management Unit for MIL-STD-1750 Processor
http://www.iaeme.com/IJECET/index.asp 47 editor@iaeme.com
INTRODUCTION
A memory management unit (MMU), sometimes called (PMMU) paged memory
management unit. It is a computer hardware component for using accesses to memory
requested by the CPU. First performing the translation of virtual memory addresses to
physical addresses (i.e., virtual memory management), it is implemented as part of the
central processing unit, but it can also be in the form of a separate integrated circuit.
MMU is clearly performing the virtual memory management, bus arbitration, memory
protection, cache control and bank switchin.
ASIC VS FPGA
Table 1 PI ASIC versus PI FPGA
Processor Interface ASIC Processor Interface FPGA
Designed for 12 MHz Designed for 24 MHz
MMU external to ASIC MMU to address up to 1M locations
Interface for Both Internal and External
Shared RAMs
Interface for Internal Shared RAM only
Employs existing EDAC for RAMs Employs Core EDAC® IP Core for Internal
RAM and existing EDAC for External RAM
Nature of Internal RAM : DP SRAM with
8K word of Memory
Nature of Internal RAM : SRAM with single
port, 8K word memory
I/O Decoding for some addresses not present I/O Decoding for all I/O addresses present
EEPROM Interface logics absent EEPROM Interface Logics provided
NEED FOR MMU
The HX-1750 processor can address a maximum of 64kB of memory through its
address lines which falls short of 1MB of memory observed on onboard. Thus, the
MMU caters to this need by providing extended addressing and making it possible for
the processor address 1 MB of memory.
DESIGN OF MMU
Memory Management Unit is implemented in RTAX-1000 FPGA. The design of
MMU is similar to that of the existing MA 31751 MMU chip [8]. The principal
function of memory management unit is to provide extended addressing to the
processor by means of address translation. The BPU of MA 31751 is not implemented
in the memory management unit design. The MMU module is designed only to
increase the memory addressing capability of the HX-1750 CPU.
The processor inputs the 16 bit Address (ADDR_VALID [15:0]) and the Address
State (AS [3:0]). The memory management unit performs address translation to output
the Physical Page Address which is referred to by Extended Address (EA [7:0]). The
extended 20 bit address is formed by the concatenation of EA [7:0] and the
ADDR_VALID [11:0] as shown in the figure 1.

3. Prasad.S.G, Dr Siva Yellampalli and Naveen.V
http://www.iaeme.com/IJECET/index.asp 48 editor@iaeme.com
Figure 1 Generation of 20 bit Physical Address
The main memory (SRAM) is divided in to 256 pages of size 4kB each.
Therefore, the MMU may have an array of registers which contain the physical page
addresses (PPA) of all the pages available within the memory.
The MMU decodes the incoming processor address to classify the command as
either an operation on Instruction registers or Operand registers.
PAGE REGISTERS
The main memory is divided into 256 pages of 4k words each. The MMU maps the
system memory into these 4k word pages. A page is a block of physical page memory
which is uniquely specified by the PPA [3]. A given address within any page is
specified by the least significant twelve bits of the CPU address bus. Each page
register is 8 bit wide and contains the physical page address of a page in the main
memory. A total of 512 page registers divided into two groups of 256 registers each,
one dedicated for Instruction memory space and one for Operand memory space.
The MMU is initialized to provide a linear, one to one mapping of the PPA when
system reset occurs. The CPU may change the mapping when it is in privileged
instruction mode using XIO commands 5100 to 52FF as defined in MIL-STD-1750.
Two register banks, one for instruction and one for operand are created. Each bank is
implemented as an 8 bit word array of length 256.
Write Operation
The processor data is written into the specified 8 bit register address (REG_ADDR).
Write operation into the file registers occurs during the rising edge of the IOWR
(write clock (W_CLK)) and when the write enable (WE) is high.
Read Operation
Two 256 to 1 MUX are placed one outside each set of page registers (instruction and
operand page registers). Depending on the selection lines (REG_ADDR), the

4. Design and Development of Memory Management Unit for MIL-STD-1750 Processor
http://www.iaeme.com/IJECET/index.asp 49 editor@iaeme.com
multiplexer selects the content of required page register and relays it to the 2 to 1
multiplexer as show in figure 2. The 2 to 1 MUX selects between the signals
EA_INSTRUCTION and EA_OPERAND depending upon the selection line
REG_ADDR to output MMU_DATA_OUT. The data for processor read is always
available on EA [7:0]. The MMU data for processor read is routed to the processor
data bus in the data bus routing module when the enable MMU_DATA_CS is
activated.
Figure 2 Read/write logic on Page registers
Translation
Two 256 to 1 multiplexers are placed one outside each set of page registers
(instruction and operand page registers). Depending on the selection lines
(TRANS_ADR), the multiplexer selects the contents of required page register and
relays it to the 2 to 1 multiplexer. TRANS_ADR to the multiplexer is formed by the
concatenation of AS [3:0] and ADDR_VALID [3:0]. MMU_TRANS_PORT always
contains the contents of the register specified by TRANS_ADR.
MMU_TRANS_PORT is either EA_INSTRUCTION or EA_OPERAND depending
upon the bank which outputs MMU_TRANS_PORT.
The 2 to 1 multiplexer in the right hand corner [9]. Depending upon the value of
the selection line DI, either the EA_INSTRUCTION or the EA_OPERAND is
selected and relayed as the required EA [7:0] as an output of the memory management
unit module.
Identical to read, the translation operation is asynchronous. EA [7:0] is always
available outside the memory management unit module irrespective of any translation
enable. To make use of EA [7:0], the processor asserts the IOM signal to logic low.
The extended 20 bit address is formed by concatenation of EA [7:0] and the
ADDR_VALID [11:0].

5. Prasad.S.G, Dr Siva Yellampalli and Naveen.V
http://www.iaeme.com/IJECET/index.asp 50 editor@iaeme.com
SIMULATION AND RESULTS
The simulated results obtained using ModelSim simulator and the device utilization
obtained using Libero Microsemi.
The VHDL code for memory management unit module was simulated for all
combinations of Data, Address and Control signals.
Figure 3 Write operation timing waveform
Figure 4 Read operation timing waveform

6. Design and Development of Memory Management Unit for MIL-STD-1750 Processor
http://www.iaeme.com/IJECET/index.asp 51 editor@iaeme.com
Figure 5 the simulated result of the MMU
SL. No Test case
1 Operand Register Read from Locations D200 to D2FF
2 Instruction Register Read from locations D100 to D1FF
3 Operand Register Write from Locations 5200 to 52FF
4 Instruction Register Write from Locations 5100 to 51FF
5 Operand Translation for TRANS_ADR values ranging from 00 to FF
6 Instruction Translate for TRANS_ADR values ranging from 00 to FF
7 Operand Register and Instruction Register Read with IOM =0
8 Operand and Instruction Translate with IOM=1
Table 2 List of FPGA Simulation Test Case
CONCLUSION AND FUTURE WORK
MMU for extending the 16 bit processor address to reference 1M words (20 bit
address) with the use of 256 Instruction and Operand page register each has been
designed, tested for implementation. This design work will optimize the board by
limiting the need for a dedicated memory management unit device and also helps to
tide over the obsolescence in the MIL-STD-1750 compatible MMU.
Microsemi RTAX1000S FPGA is used to realize the logics. Design and
implementation of the MMU will be done in Microsemi’s Libero environment using
VHDL language.

7. Prasad.S.G, Dr Siva Yellampalli and Naveen.V
http://www.iaeme.com/IJECET/index.asp 52 editor@iaeme.com
Future work can integrate the Block Protection Unit (BPU) for memory protection
into the design as with the case of the dedicated Memory Management Unit.
REFERENCES
[1] ISRO ASIC /FPGA design and development guidelines.
[2] John L. Hennsley and David A. Patterson, “Computer Architecture – A
quantitative approach”, 2 edition Morgan Kaufmanns publishers inc. San
Francisco, California, 1996.
[3] Memory management overview, Karl Ingström, Anders Daleby. Department of
Computer Engineering, University of Mälardalen, Sweden, 2001.
[4] K.C. Chang, “Digital Design & Modeling with VHDL & Synthesis” IEEE
Computer society press.
[5] Radiation-Hardended FPGAs Datasheet -Actel Corporation. See the Actel
website for the latest version of the datasheet. V3.1. Radiation-Hardened FPGAs.
Features www.actel.com/documents/RadHard_DS.pdf.
[6] Processor Interface ASIC (PI-ASIC) design document.
[7] Stefan Sjoholm, “VHDL for Designers” Pearson Prentice Hall.
[8] MA31751 -User configurable, the MA31751 can perform as an MMU, a BPU or
both MMU and BPU, conforming to MA31751 devices can be used to give 16M
wordsoflogical.www.dynexsemi.com/assets/
SOS/Datasheets/DNX_MA31751_Jul02.pdf.
[9] Operating system concepts, 4ed, A. Silberschatz and P.B Galvin, ISBN 0-201-
50480-4, 1994, Addison-Wesley.
[10] J.K. Kishore, “A memory management unit for satellite recovery experiment”,
International Astronautical Congress 2007.
[11] R. Revathi S. Sinthuja Dr. N. Manoharan and N. Rajendiran, Allocation of Power
in Relay Networks for Secured Communication, International Journal of
Advanced Research in Engineering & Technology, 6(8), 2015, pp. 135–144.
[12] Dhanya Pushkaran and Neethu Bhaskar, AES Encryption Engine for Many Core
Processor Arrays for Enhanced Security, International Journal of Electronics and
Communication Engineering & Technology, 5(12), 2014, pp. 106–111.