Computer Architecture Assignment: Raspberry Pi, Memory, and Buses

This assignment delves into the fundamentals of computer architecture, using the Raspberry Pi as a case study. Task 1 dissects the Raspberry Pi, differentiating between its internal and external components with visual aids. Task 2 examines various types of internal memory, including RAM, ROM, and cache, analyzing their characteristics and impact on system performance, with specific reference to the Raspberry Pi's memory capabilities. Task 3 focuses on the system bus, detailing the interconnection of address, data, and control buses, and explaining their respective functions while discussing how bus width affects machine performance. Finally, Task 4 explores CPU registers and instructions, comparing Motorola 68000 and ARM architectures, providing examples of their instruction sets and their interpretations. The assignment aims to provide a comprehensive understanding of computer architecture principles.

Task 1
Raspberry pi
Raspberry pi is a portable and a single board computer, small in size (like that of a credit card),
and performs a range of tasks just like desktop computer. Raspberry pi is used for browsing,
playing video games, playing HD videos, processing word documents and making spreadsheets
(Upton, 2014). Raspberry pi has two models, model A and model B, the difference being on the
cost, Ethernet and USB ports. Figure 1 shows the architecture of raspberry pi. The components
can be decomposed into two parts, internal and external components.
Figure 1. Raspberry pi architecture
Internal components
Memory- raspberry pi program memory (RAM) varies with the model. First release of Model A
had 256MB of SDRAM whereas Model B had 51MB of the memory. The RAM memory is made
up of splits, allocated for CPU and GPU. For the normal personal computer, RAM memory
capacity is in gigabytes. Other releases of raspberry pi however have memory more than
256MB.
Central processing unit (CPU)- is the brain of the personal computer and is tasked to execute
instructions of the raspberry pi through logical operations. First generation raspberry pi had
Broadcom BCM2835 System on chip, with 700MHz ARM11 processor. Generally this type of
computer uses ARM11 series of processors.
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Graphics processing unit (GPU)- this is a chip in the board responsible for speeding up
mathematical calculations that results in image, video and animations creation. Raspberry pi
GPU uses ‘Broadcom video core IV’ (Maksimovic & Vujovic, 2014 ).
External components
Ethernet port- this is the main gateway port of the raspberry pi used for communication with
external devices. This is the port that is used to connect the home router for internet access.
LEDs- the light emitting diodes are used to indicate status of certain operations by a light in the
raspberry pi board
USB- USB is used to connect all types of peripheral devices to the raspberry pi for example
keyboard, mouse etc. Model A of the raspberry pi has one USB port and model B has two ports.
USB port can also be used in expanding the number of ports.
UART- abbreviation for Universal Asynchronous Receiver and Transmitter is the output and
input port of the raspberry pi. This port is used for serial data transfer in form of text. It also
used to convert debugging code
HDMI- this allows the user to attach high definition TV and other LCD devices to the raspberry
pi
SD card slot-SD card with operating system installed in it, to be used by the raspberry pi is
inserted through the slot
Power connector- 5 volts USB connector is used for supplying external power source into the
raspberry pi.
GPIO Pins- they are used for communication with other electronic boards. GPIO pins accepts
output and input commands as programmed in raspberry pi
Task 2
Memory is a device used for storing data and programs either temporarily or permanently, to
be used by a computer or other electronic devices (stalling, 2013). In computer systems there
are two types of memory, namely external and internal memory. Internal memory is where
small quantity of data is temporarily stored and can be directly and easily accessed by the CPU.
Most of the internal storage devices are volatile meaning upon power loss the data is lost.
There are three main types of internal memory namely RAM cache and ROM.
Random Access Memory
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This is the memory that holds data and programs that are in use while the computer the
operating so as to increase the speed at which it is accessed. It acts as intermediary between
the CPU and hard disk and this helps speed up the operation of a computer. Reading data in
RAM is faster that that stored in external memory. CPU is responsible for overwriting old data
in the RAM when full, so as not to slow down its operations. Its main characteristics are
 It’s a volatile memory
 Its expensive
 RAM reads and writes data at byte level
RAM comes in two types, static RAM and dynamic RAM. The differences are outlined in the
table 1
Table 1: DRAM versus SRAM
Dynamic RAM Static RAM
Its technology is based on capacitors Its technology is based on flip-flop
Used where small memory capacity is required Used where large memory capacity is required
Slower access time Faster access time
Read Only Memory
The memory is not volatile, found in chips and is primarily used for storing computer bios along
with other vital information. Its major characteristics are as shown below
 Data is written into it when manufacturing
 Once it has been programmed, ROM can never be erased
 ROM contents are only fixed during fabrication by metal masks
ROM is widely used in storing control programs, for permanent data storage, BIOS, embedded
system code among others
ROM comes in three types, EEPROM, EPROM and PROM
Cache memory
Cache memory is the fastest memory of the three main types acting as a buffer between
Random Access Memory and the central processing unit as shown in figure 2. Cache memory
stores data and instructions that are frequently used or requested by CPU so that they are
readily available when requested (Stalling 2014). General impact of cache memory is a
reduction in time used to access data from RAM.
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Figure 2: cache memory
Raspberry pi uses RAM type of memory and the SD card carries the operating system to be
used. The first model had 256MB but with new models released as time goes, superior models
are realized with latest model having a large capacity memory of 1GB.
Task 3
Data bus, address and control buses
The three buses forms system bus and interacts with the memory, CPU and peripherals as
shown figure 3
Figure 3: interconnection of system bus
Data bus transfers data in and out of CPU and is always called bidirectional bus. Data bus also
performs read and write operations as the CPU ‘writes’ data into the data bus, and also ‘read’
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data from memory locations. Data bus has eight, sixteen or thirty two lines, and the number of
these parallel lines is the same as the size of a word to be written or read. So 32 bit CPU will
perform better than 8 and 16 bit.
Address bus is used in read and write operations to specify a location or physical address. Say
the CPU has to write or read from or to the memory, the CPU will utilize the address bus to help
it locate specific memory block it wants to access (Ishizu, Furutani & Sato, 2016). The width of
the address bus is determined by the size of the memory. For instance, if the width of address
bus is 8 bits, 2^6 memory locations will be address which is similar to 256MB memory space,
and width of 16 bits address 4GB of memory space.
Control bus is used by central processing unit to communicate with the memory and
peripherals with an aid of control signals.
Raspberry pi uses ARM CPU, some models such as B+ uses ARM cortex and its bus system is as
illustrated figure 4 (Pi, 2014).
Figure 4: ARM cortex bus system
TASK 4
Motorola 68000 CPU register sets
This processor has a total of eighteen registers and are as follows
 One Status Register (SR) which consists of the Condition Code Register (CCR) and System
Byte
 Eight data registers numbered from D0 to D7
 One Program Counter (PC).
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 Eight address registers numbered from A0 to A7
Motorola 68000 Instructions
The following are few examples of instruction and their interpretation
CMP #X, D1- value of X is subtracted from contents of D1 and the result discarded
SUB #X, D3- value of X is subtracted from D3 and the result stored in that register
MOVE #X, D5- loads D5 with value X
CMP D2,D4- contents of D4 subtracted from contents of D2 and the result discarded.
MOVE D7, L- copy contents of D7 into memory location L.
Raspberry pi’s ARM register set.
ARM CPU has thirty 32 bit registers highlighted below
 One Program Counter r15
 Thirteen general purpose data registers numbered from r0-r12
 One Special Purpose Program Status Register, (xPSR)
 One Stack Pointer (SP)
 One Link Register (LR) r14
Raspberry pi’s ARM instructions
Few examples of ARM CPU instructions and their interpretation (Brock & Bruce, 2013)
ADD r0, r3, r6 - contents of operand3 (r3) is added to operand6 (r6) and the result is stored in
r0
ADD r2, r4, #X - value of X is added to contents of operand4 (r4) and the result is stored in r2
MOVELE r0, #X - it moves number X to r0 only if less than or equal (LE) condition is satisfied.
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REFERENCES
Brock, J.D., and Bruce, R.F. 2013. Changing the world with a Raspberry Pi. Journal of Computing
Sciences in Colleges, 29(2), pp.151-153.
Ishizu, T., Furutani, K. and Sato, K.,2016. Semiconductor device, central processing unit, and
electronic device. U.S. Patent Application 15/011,809.
Maksimović, M.,and Vujović, V. 2014. Raspberry Pi as Internet of things hardware:
performances and constraints. design issues, 3(8).
Pi, R., 2015. Raspberry Pi Model B.
Stalling.W, 2013, Computer Organization Architecture: Designing for performance,10th edition,
New York, Pearson,
Upton, E. and Halfacree, G., 2014. Raspberry Pi user guide. John Wiley & Sons.
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