Bus-Based Computer
Systems
Designing with microprocessors.
Development and debugging.
System-level performance analysis.
© 2008 Wayne Wolf
Overheads for Computers as
Components 2nd ed.
System architectures
Architectures and components:
software;
hardware.
Some software is very hardwaredependent.
© 2008 Wayne Wolf
Overheads for Computers as
Components 2nd ed.
Hardware platform
architecture
Contains several elements:
CPU;
bus;
memory;
I/O devices: networking, sensors,
actuators, etc.
How big/fast much each one be?
© 2008 Wayne Wolf
Overheads for Computers as
Components 2nd ed.
Software architecture
Functional description must be broken into
pieces:
division among people;
conceptual organization;
performance;
testability;
maintenance.
© 2008 Wayne Wolf
Overheads for Computers as
Components 2nd ed.
Hardware and software
architectures
Hardware and software are intimately
related:
software doesn’t run without hardware;
how much hardware you need is
determined by the software requirements:
speed;
memory.
© 2008 Wayne Wolf
Overheads for Computers as
Components 2nd ed.
Evaluation boards
Designed by CPU manufacturer or others.
Includes CPU, memory, some I/O devices.
May include prototyping section.
CPU manufacturer often gives out
evaluation board netlist---can be used as
starting point for your custom board
design.
© 2008 Wayne Wolf
Overheads for Computers as
Components 2nd ed.
Adding logic to a board
Programmable logic devices (PLDs)
provide low/medium density logic.
Field-programmable gate arrays (FPGAs)
provide more logic and multi-level logic.
Application-specific integrated circuits
(ASICs) are manufactured for a single
purpose.
© 2008 Wayne Wolf
Overheads for Computers as
Components 2nd ed.
The PC as a platform
Advantages:
cheap and easy to get;
rich and familiar software environment.
Disadvantages:
requires a lot of hardware resources;
not well-adapted to real-time.
© 2008 Wayne Wolf
Overheads for Computers as
Components 2nd ed.
Typical PC hardware
platform
CPU
memory
CPU bus
intr
ctrl
DMA
controller
bus
interface
device
high-speed bus
timers
bus
interface
low-speed bus
device
© 2008 Wayne Wolf
Overheads for Computers as
Components 2nd ed.
Typical busses
PCI: standard for high-speed interfacing
33 or 66 MHz.
PCI Express.
USB (Universal Serial Bus), Firewire (IEEE
1394): relatively low-cost serial interface
with high speed.
© 2008 Wayne Wolf
Overheads for Computers as
Components 2nd ed.
Software elements
IBM PC uses BIOS (Basic I/O System) to
implement low-level functions:
boot-up;
minimal device drivers.
BIOS has become a generic term for the
lowest-level system software.
© 2008 Wayne Wolf
Overheads for Computers as
Components 2nd ed.
Example: StrongARM
StrongARM system includes:
CPU chip (3.686 MHz clock)
system control module (32.768 kHz clock).
•
•
•
•
•
•
© 2008 Wayne Wolf
Real-time clock;
operating system timer
general-purpose I/O;
interrupt controller;
power manager controller;
reset controller.
Overheads for Computers as
Components 2nd ed.
Debugging embedded
systems
Challenges:
target system may be hard to observe;
target may be hard to control;
may be hard to generate realistic inputs;
setup sequence may be complex.
© 2008 Wayne Wolf
Overheads for Computers as
Components 2nd ed.
Host/target design
Use a host system to prepare software for
target system:
target
system
host system
© 2008 Wayne Wolf
serial line
Overheads for Computers as
Components 2nd ed.
Host-based tools
Cross compiler:
compiles code on host for target system.
Cross debugger:
displays target state, allows target system to
be controlled.
© 2008 Wayne Wolf
Overheads for Computers as
Components 2nd ed.
Software debuggers
A monitor program residing on the target
provides basic debugger functions.
Debugger should have a minimal footprint
in memory.
User program must be careful not to
destroy debugger program, but , should
be able to recover from some damage
caused by user code.
© 2008 Wayne Wolf
Overheads for Computers as
Components 2nd ed.
Breakpoints
A breakpoint allows the user to stop
execution, examine system state, and
change state.
Replace the breakpointed instruction with
a subroutine call to the monitor program.
© 2008 Wayne Wolf
Overheads for Computers as
Components 2nd ed.
ARM breakpoints
0x400
0x404
0x408
0x40c
MUL r4,r6,r6
ADD r2,r2,r4
ADD r0,r0,#1
B loop
uninstrumented code
© 2008 Wayne Wolf
0x400
0x404
0x408
0x40c
MUL r4,r6,r6
ADD r2,r2,r4
ADD r0,r0,#1
BL bkpoint
code with breakpoint
Overheads for Computers as
Components 2nd ed.
Breakpoint handler actions
Save registers.
Allow user to examine machine.
Before returning, restore system state.
Safest way to execute the instruction is to
replace it and execute in place.
Put another breakpoint after the replaced
breakpoint to allow restoring the original
breakpoint.
© 2008 Wayne Wolf
Overheads for Computers as
Components 2nd ed.
In-circuit emulators
A microprocessor in-circuit emulator is a
specially-instrumented microprocessor.
Allows you to stop execution, examine
CPU state, modify registers.
© 2008 Wayne Wolf
Overheads for Computers as
Components 2nd ed.
Logic analyzers
A logic analyzer is an array of low-grade
oscilloscopes:
© 2008 Wayne Wolf
Overheads for Computers as
Components 2nd ed.
Logic analyzer
architecture
sample
memory
UUT
system clock
microprocessor
vector
address
controller
clock
gen
state or
timing mode
© 2008 Wayne Wolf
keypad
Overheads for Computers as
Components 2nd ed.
display
Boundary scan
Simplifies testing of
multiple chips on a
board.
Registers on pins can
be configured as a
scan chain.
Used for debuggers,
in-circuit emulators.
© 2008 Wayne Wolf
Overheads for Computers as
Components
How to exercise code
Run on host system.
Run on target system.
Run in instruction-level simulator.
Run on cycle-accurate simulator.
Run in hardware/software co-simulation
environment.
© 2008 Wayne Wolf
Overheads for Computers as
Components 2nd ed.
Debugging real-time code
Bugs in drivers can cause nondeterministic behavior in the foreground
problem.
Bugs may be timing-dependent.
© 2008 Wayne Wolf
Overheads for Computers as
Components 2nd ed.
System-level performance
analysis
Performance depends
on all the elements of
the system:
CPU.
Cache.
Bus.
Main memory.
I/O device.
© 2008 Wayne Wolf
memory
CPU
cache
Overheads for Computers as
Components 2nd ed.
Bandwidth as performance
Bandwidth applies to several components:
Memory.
Bus.
CPU fetches.
Different parts of the system run at
different clock rates.
Different components may have different
widths (bus, memory).
© 2008 Wayne Wolf
Overheads for Computers as
Components 2nd ed.
Bandwidth and data
transfers
Video frame: 320 x 240 x 3 = 230,400
bytes.
Transfer in 1/30 sec.
Transfer 1 byte/msec, 0.23 sec per frame.
Too slow.
Increase bandwidth:
Increase bus width.
Increase bus clock rate.
© 2008 Wayne Wolf
Overheads for Computers as
Components 2nd ed.
Bus bandwidth
T: # bus cycles.
P: time/bus cycle.
Total time for
transfer:
O1
D
O2
W
t = TP.
D: data payload
length.
O1 + O2 = overhead
O.
© 2008 Wayne Wolf
Tbasic(N) = (D+O)N/W
Overheads for Computers as
Components 2nd ed.
Bus burst transfer
bandwidth
T: # bus cycles.
P: time/bus cycle.
Total time for
transfer:
1
2
B
…
O
W
t = TP.
D: data payload
length.
O1 + O2 = overhead
O.
© 2008 Wayne Wolf
Tburst(N) = (BD+O)N/(BW)
Overheads for Computers as
Components 2nd ed.
Memory aspect ratios
16 M
64 M
8M
1
© 2008 Wayne Wolf
4
Overheads for Computers as
Components 2nd ed.
8
Memory access times
Memory component access times comes
from chip data sheet.
Page modes allow faster access for
successive transfers on same page.
If data doesn’t fit naturally into physical
words:
A = [(E/w)mod W]+1
© 2008 Wayne Wolf
Overheads for Computers as
Components 2nd ed.
Bus performance
bottlenecks
Transfer 320 x 240
video frame @ 30
frames/sec = 612,000
bytes/sec.
Is performance
bottleneck bus or
memory?
© 2008 Wayne Wolf
memory
Overheads for Computers as
Components 2nd ed.
CPU
Bus performance
bottlenecks, cont’d.
Bus: assume 1 MHz bus, D=1, O=3:
Tbasic = (1+3)612,000/2 = 1,224,000 cycles
= 1.224 sec.
Memory: try burst mode B=4, width
w=0.5.
Tmem = (4*1+4)612,000/(4*0.5) = 2,448,000
cycles = 0.2448 sec.
© 2008 Wayne Wolf
Overheads for Computers as
Components 2nd ed.
Performance spreadsheet
bus
clock period
W
D
O
N
T_basic
t
© 2000 Morgan
Kaufman
1.00E-06
2
1
3
612000
1224000
1.22E+00
memory
clock period
W
D
O
B
N
1.00E-08
0.5
1
4
4
612000
T_mem
t
2448000
2.45E-02
Overheads for Computers as
Components
Parallelism
Speed things up by
running several units
at once.
DMA provides
parallelism if CPU
doesn’t need the bus:
DMA + bus.
CPU.
© 2008 Wayne Wolf
Overheads for Computers as
Components 2nd ed.