CS 630: Advanced Microcomputer Programming

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CS 630 Advanced Microcomputer Programming

• Fall 2008
• Professor Allan B. Cruse
• University of San Francisco

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Course Synopsis

• We study Intel-64 processor architecture
• Its implemented in our Core-2 Quad CPU
• We pretend were using a bare machine (i.e. no
  operating system to hide whats going on, just
  standard PC hardware and accompanying
  vendor-supplied firmware)
• So we get to build our own miniature OS
• Doing this will bring us face-to-face with the
  CPUs most fundamental capabilities

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Methodology

• Our interactive computer classroom lets us take a
  hands on approach to our studies (i.e., we
  combine theory with practice)
• Typically well devote first part each class to a
  lecture about aspects of x86 theory
• Then well take time in the second part of class
  for laboratory exercises that put the newly
  learned ideas into working code

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Course prerequisites

• Experience with C / C programming
• Familiarity with use of Linux / UNIX OS
• Acquaintance with x86 assembly language
• Knowledge of the x86 general registers
• Awareness of the x86s instruction-set
• Understand the CPUs fetch-execute cycle
• Recall the ways memory is addressed

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Simplified component diagram
Central Processing Unit
Main Memory
system bus
I/O device
I/O device
I/O device
I/O device

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Review of the legacy x86 API
CS
EAX
DS
EBX
ES
ECX
FS
EDX
GS
ESI
SS
EDI
Segment Registers (16-bits)
EBP
ESP
EIP
General Registers (32-bits)
EFLAGS
Program Control and Status Registers (32-bits)
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Review of Instruction-Set

• Data-transfer instructions (mov, xchg, )
• Control-transfer instructions (jmp, call, )
• Arithmetic/Logic instructions (add, or, )
• Shift/Rotate instructions (shr, rol, )
• String-manipulation instructions (movs, )
• Processor-control instructions (cli, hlt, )
• Floating-point instructions (fldpi, fmul, )

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Review Fetch-Execute Cycle
main memory
central processor
Temporary Storage (STACK)
ESP
Program Variables (DATA)
EAX
EAX
EAX
EAX
Program Instructions (TEXT)
EIP
the system bus
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Steps in Fetch-Execute Cycle
Fetch next instruction
Advance instruction-pointer
Decode fetched instruction
Execute decoded instruction
INTR ?
Interrupt Service Routine
no
yes
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Review of operand addressing

• Implicit addressing (e.g. pushf, cbw, scasb,
  cli, xlat, )
• Register addressing (e.g., mov ax, bx)
• Direct addressing (e.g., incl salary, movw
  0, counter, )
• Indirect addressing (e.g., add dx,
  0x14(ebx, esi, 2) )

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Course Textbook

• Tom Shanley, Protected Mode Software
  Architecture, Addison-Wesley (1996)
• Initial reading assignment
• Week 1 Read Part One (Chapters 1-3)
• Week 2 Read Part Two (Chapters 4-5)

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Instructor Contact Information

• Office Harney Science Center 212
• Hours Mon-Wed-Fri 1230pm-115pm and
  Tues-Thurs 630pm-715pm
• Phone [phone]
• Email cruse_at_usfca.edu
• Webpage lthttp//cs.usfca.edu/crusegt

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CPU Execution Modes
POWER-ON / RESET
REAL MODE
PROTECTED MODE
VIRTUAL 8086 MODE
SYSTEM MANAGEMENT MODE
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The pre-boot environment

• None of the normal library functions
• No graphical desktop, no file-system
• No editors, compilers, debuggers
• No network-access, no mouse, no printer
• Only one of the four processors is active
• Only a tiny fraction of the system memory is
  accessible (only 1-MB, out of 4096-MB)
• The method of addressing memory is very different
  from what were accustomed to!

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64KB Memory-Segments

• Fixed-size segments (can be overlapping)
• Segments start on paragraph boundaries
• Segment-registers serve as selectors

stack
data
SS
DS
code
CS
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Real-Mode Address-Translation
16-bit segment-address
16-bit offset-address
0x1234
0x6789
Logical address
0x12340 0x06789 ---------------- 0x18AC9

x 16

20-bit bus-address
0x18AC9
Physical address
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Using ROM-BIOS functions

• Our system firmware provides many basic
  service-functions that real mode programs can
  invoke (this includes boot-loaders)
• Video display functions
• Keyboard input functions
• Disk access functions
• System query functions
• A machine re-boot function

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A valuable Online Reference

• Professor Ralf Browns Interrupt List (see
  webpage link under Resources)
• It tells how to make BIOS system-calls, to
  perform numerous low-level services from within
  Real-Mode 8086 applications (such as boot
  loader programs)

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Power-On
CSIP
ROM-BIOS
Expansion ROMs
Video BIOS
VRAM
DRAM
1-MB
uninitialized memory area
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System setup
ROM-BIOS
CSIP
Expansion ROMs
Video BIOS
VRAM
Extended BIOS Data Area
DRAM
EBDA
1-MB
ROM-BIOS DATA AREA
Interrupt Vector Table
RBDA
IVT
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Bootstrap Loader
ROM-BIOS
Expansion ROMs
Video BIOS
VRAM
Extended BIOS Data Area
DRAM
EBDA
1-MB
CSIP
BOOT_LOCN
ROM-BIOS DATA AREA
Interrupt Vector Table
RBDA
Disk Storage
IVT
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A very short example
// smile.s .section .text our linker needs
this name mov 0x0E, ah BIOS
function-selector mov 0x01, al
character-glyph selector mov 0x00, bh
display-page selector int 0x10 invoke
ROM-BIOS service freeze jmp freeze enter an
infinite loop .org 510 offset to
boot-signature .byte 0x55, 0xAA value for
boot-signature ,end nothing more to assemble
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Assemble, link, and install
Use the GNU/linux assembler to translate
source-code to object-code as
smile.s -o smile.o Use the GNU/Linux linker
to convert object-code to binary-format ld
smile.o -T ldscript -o smile.b NOTE This
linking step requires using a special
linker-script in order to override the
default ELF-format output-file (the customary
format of a file that the Linux operating
system knows how to load and execute) Copy the
binary-executable to the place on our CS630
disk-partition where the GRUB boot-loader will
expect to find it dd ifsmile.b
of/dev/sda4
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Our fileview utility

• You can use the fileview.cpp program (on our
  cs630 course-website) as a convenient tool for
  viewing files ./fileview smile.b
• Since fileview also works with device-files (if
  you have the required read-permission), you can
  verify that smile.b is successfully installed
  on our CS630 disk-partition
• ./fileview /dev/sda4

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Observations

• Our smile.s program-code does not make use of
  any assembly-language labels, nor does it use any
  instructions that would be differently translated
  for the real-mode pre-boot execution
  environment than for the protected-mode
  environment used by Linux application-programs
• A few different coding-conventions would be
  needed when these conditions change

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Example

• Any assembly-language instruction that refers to
  a 16-bit (or to a 32-bit) register will need to
  be assembled differently for real-mode
  execution
• This is accomplished using the .code16 assembler
  directive

mov 0x1301, ax inserts an operand-size
override prefix .code16 needed for correct
real-mode execution mov 0x1301, ax omits
the operation-size override prefix
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Symbolic addresses

• The linker assumes your code will reside in
  memory at an address-offset equal to 0, so it
  assigns address-values to all of your
  program-symbols accordingly
• But the bootstrap-loader places your code at an
  address-offset equal to 0x7C00 !
• Thus you must perform a renormalizing operation
  if you want to use your symbols

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Example that uses symbols
.code16 for x86 real-mode
.section .text ljmp 0x07C0, main (this
renormalize CSIP) main mov cs, ax address
program data mov ax, ds with DS
register mov ax, es also ES
register mov msg, bp point ESBP to
string mov len, cx string-length into
CX mov 0x0009, bx page and color in
BX mov 0x0A28, dx row and column in
DX mov 0x1301, ax write_string
function int 0x10 invoke BIOS
service freeze jmp freeze enter an infinite
loop msg .ascii Hello, world! \n
text-message to display len .short . msg
length of the message
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Effect of the long-jump
BOOT_CODE
BOOT_CODE
IP 0x0005
IP 0x7C00
CS 0x07C0
Now all the symbol offsets are correct, relative
to segment register CS
CS 0x0000
AFTER
BEFORE
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In-class exercise 1

• Download the textfile welcome.s from our class
  website into your own subdirectory
• cp /home/web/cruse/cs630/welcome.s .
• Then assemble it (use as), link it (use ld)
  and install it (use dd) on your hard disks
  partition
• Reboot your computer, and select the GRUB
  menu-option which will execute that code
• Did you see the welcome-message? Were you able
  to reboot by simply pressing a key?

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In-class exercises 2, 3, 4

• Can you modify the welcome message so that is
  will also include your name? Can you change the
  color from green to red? Can you make the
  message appear near the bottom of the console
  screen?