CS 630: Advanced Microcomputer Programming

Fall 2006

Professor Allan B. Cruse

University of San Francisco

Course Synopsis

• We study the IA32 processor architecture

• It’s implemented in our Pentium 4 CPUs

• Also implemented in some earlier CPUs

• Not only Intel, but also by its competitors (e.g., present as ‘legacy mode’ in AMD64)

• IA32 architecture adopted by newer Macs

• IA32 architecture continues in Core 2 Duo

Point-of-View

• For study purposes we can pretend we’re studying a ‘bare machine’ (i.e., it just has standard PC hardware for doing I/O, and ROM-BIOS firmware supplied by vendor, but lacks any operating system software).

• So we get to ‘build our own’ miniature OS

• Doing this will bring us face-to-face with the CPU’s most fundamental capabilities

Methodology

• Our interactive computer classroom lets us take a ‘hands on’ approach to our studies (i.e., we combine ‘theory’ with ‘practice’)

• Typically we’ll devote first part each class to a ‘lecture’ about aspects of IA32 theory

• Then we’ll take time in the second part of class for ‘laboratory exercises’ that put the newly learned ideas into ‘working code’

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 x86’s instruction-set

• Understand the CPU’s fetch-execute cycle

• Recall the ways memory is addressed

Review of System Components

CentralProcessing

Unit

MainMemory

I/Odevice

I/Odevice

I/Odevice

I/Odevice

system bus

Review of the x86 API

EAX

EBX

ECX

EDX

ESI

EDI

EBP

ESP

General Registers (32-bits)

CS

DS

ES

FS

GS

SS

Segment Registers (16-bits)

EIP

EFLAGS

Program Control and Status Registers (32 bits)

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, …)

Review “Fetch-Execute” Cycle

ESP

EIPProgram

Instructions(TEXT)

ProgramVariables(DATA)

TemporaryStorage(STACK)

main memory

central processor

EAXEAXEAXEAX

the system bus

Steps in ‘Fetch-Execute Cycle’

INTR?

Fetch next instruction

Advance instruction-pointer

Decode fetched instruction

Execute decoded instruction

noInterruptServiceRoutine

yes

Review of operand addressing

• Implicit addressing (e.g. pushf, cbw, scasb, cli, xlat, …)

• Direct addressing(e.g., incl salary, movw $0, counter,

…)

• Indirect addressing(e.g., add %dx, 0x14(%ebx, %esi, 2) )

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)

Instructor Contact Information

• Office: Harney Science Center – 212

• Hours: Mon-Wed-Fri 12:30pm-1:15pm and Tues-Thurs 6:15pm-7:15pm

• Phone: (415) 422-6562

• Email: cruse@usfca.edu

• Webpage: <http://cs.usfca.edu/~cruse>

CPU Execution Modes

REALMODE

PROTECTEDMODE

VIRTUAL8086

MODE

SYSTEMMANAGEMENT

MODE

POWER-ON / RESET

Early Intel Processors

• 1971: 4004 (first 4-bit processor)

• 1972: 8008 (first 8-bit processor)

• 1974: 8080 (widely used by CP/M)

• 1978: 8086/8088 (first 16-bit processor)

• 1982: 80286: (introduced protected mode)

• 1985: 80386: (first 32-bit processor)

• 1989: 80486: (integrated floating-point)

Later Intel Processors

• 1993: Pentium processor (dual CPUs)• 1995: Pentium Pro (for high-end servers)• 1996: Pentium II (single-edge connector)• 1998: Pentium II Xeon (multiple CPUs)• 1999: Celeron (stripped down Pentium II)• 1999: Pentium III (1GHz, 512K L2 cache) • 1999: Pentium III Xeon (high-end servers)• 2001: Pentium 4 (new SIMD instructions)

Even newer Intel Processors

• 2003: Pentium-M (‘mobile’ -- for laptops)

• 2005: Pentium-D (‘dual core’ -- for ‘smp’)

• 2006: Core 2 Duo (released this summer)

• Newest CPUs support ‘EM64T’ and ‘VT’– EM64T: Extended Memory 64-bit Technology– VT: Intel’s ‘Virtualization Technology’

Backward Compatibility

• From its first commercial success onward, “backward compatibility” (i.e., support for the software legacy) has been viewed by Intel as an engineering design imperative

• So the first 16-bit processors (8086/8088), used in IBM-PCs, were designed in a way that would let them run the vast number of CP/M programs written for 8-bit 8080 CPU

Real Mode

• 8086/8088 had only one execution mode

• It used “segmented” memory-addressing

• Physical memory on 8086 was subdivided into overlapping “segments” of fixed-size

• The length of any “segment” was 64KB, to match the size of an 8080s address-space

• This scheme supported CP/M applications

• (Our Pentium CPUs continue that support)

64KB Memory-Segments

• Fixed-size segments partially overlap

• Segments start on paragraph boundaries

• Segment-registers serve as “selectors”

code

data

stack

CS

DS

SS

Real-Mode Address-Translation

0x1234 0x6789Logical address:

16-bit segment-address 16-bit offset-address

x 16 +

0x18AC9

20-bit bus-address

Physical address:

0x12340+ 0x06789

---------------- 0x18AC9

Protected Mode

• Any Pentium CPU starts up in ‘Real Mode’ • While in real mode, its behavior is like an 8086

(i.e., any program can do anything it wants, as the CPU’s protection mechanisms are disabled)

• But software can enter ‘protected mode’ (on a 80286 or higher) using a special instruction to modify a bit within a processor control-register

• Once in protected mode, the segment-sizes can be adjusted, accesses to physical memory (or to peripheral devices) can be restricted, and tasks can be isolated from interfering with one another

Enabling Protection

NE

ET

TS

EM

MP

PE

15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

80286 Machine Status Word

PE (Protection Enabled) 0=no, 1=yes

smsw %axor $1, %axlmsw %ax

Code-fragment that enables protection

Protected-Mode Segments

• Segments can have varying lengths

• Segments may, or may not, overlap

• Segments are assigned ‘access-attributes’

code

data

stack

operating system

CS

DS

SS

GS

Our ‘bare machine’

• If we want to do a “hands on” study of our CPU, without any operating system getting in our way, we have to begin by exploring ‘Real Mode’ (it’s the CPU’s startup state)

• We will need to devise a mechanism by which our program-code can get loaded into memory (since we won’t have an OS)

• This means we must write a ‘boot loader’

What’s a ‘boot loader’

• A ‘boot loader’ is a small program that is resident in the starting sector of a disk (or tape or other non-volatile storage medium)

• After testing and initializing the machine’s essential hardware devices, the startup program in the ROM-BIOS firmware will read the ‘boot loader’ into memory, at an assigned location, and then jump there

PC ROM-BIOS BOOT_LOCN

BOOT_LOCN0x00007C00

0x00007E00512 bytes

ROM-BIOS

VRAM

IVT and BDA

8086 memory-map

RAM

Vendor’s Firmware

Video Display MemoryNo installed memory

Volatile Program Memory 1-MB

Some Requirements

• A ‘boot loader’ has to be 512 bytes in size (because it has to fit within a disk sector)

• Must begin with executable machine-code• Must end with a special ‘boot signature’• Depending on the type of storage medium,

it may need to share its limited space with certain other data-structures (such as the ‘partition table’ on a hard disk, or the Bios Parameter Block’ on a MS-DOS diskette)

Writing a ‘boot loader’

• Not practical to use a high-level language

• We need to use 8086 assembly language (our classroom/lab systems provides ‘as’)

• This assembler’s syntax differ’s from the standard set by Intel and Microsoft, but it follows a tradition, established in 1970s at AT&T, for its original versions of UNIX

• That ‘as’ syntax is documented online

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

Example: Write_String function

• Setup parameters in designated registers– AH = function ID-number (e.g. 0x13)– AL = cursor handling method (e.g. 0x01)– BH = display page-number (e.g., 0x00)– BL = color attributes (e.g., 0x0A) – CX = length of the character-string – DH, DL = row-number, column-number– ES:BP = string’s starting-address (seg:off)

• Call BIOS via software interrupt (int-0x10)

Downloading a class demo

• You can ‘download’ a program source-file from our CS 630 course-website to your own ‘present working directory’ by using the Linux file-copy command, like this:

$ cp /home/web/cruse/cs630/bootmsw.s .

(Here the final period-character (‘.’) is the Linux shell’s symbol for your ‘current directory’).

Compiling and Installing

• Compiling our ‘boot loader’ using ‘as’ is a two-step operation (and requires use of a linker-script, named ‘ldscript’):

$ as bootload.s –o bootload.o$ ld bootload.o –T ldscript –o

bootload.b

• Installing our bootloader into the starting sector of a floppy diskette is very simple:

$ dd if=bootload.b of=/dev/fd0

No floppy drive!

• Our workstations no longer have diskette-drives, but we have devised alternatives:– Copy the bootloader to a hard disk partition– Install the bootloader on a diskette-image file

• Tonight we can use the first alternative: $ dd if=bootloader.b of=/dev/sda4

• The ‘grub’ menu includes an option that will let you ‘boot’ from this ‘cs630 partition’

Executing a ‘boot-loader’

• You need to perform a system ‘reboot’

• Our classroom machines will load GRUB (the Linux GRand Unified Boot-loader)

• GRUB will display a menu of Boot Options

• You can choose ‘boot from a disk-partition’

• Or you can boot from a diskette-image file

In-class Exercise #1

• Look at our CS 630 class website: <http://cs.usfca.edu/~cruse/cs630>

• Download, assemble, and install our demo‘bootmsw.s’

• Copy the ‘binary-executable’ (i.e., bootmsw.b’) to the first sector of the hard-disk’s partition #4:

$ dd if=bootmsw.b of=/dev/sda4

• Reboot machine and use GRUB’s menu to boot our demo-program from the ‘cs630 partition’

In-class Exercise #2

• Now modify our demo so it will permit a user to ‘reboot’ just by pressing any key

• This exercise will require you to edit your copy of our demo-program’s source-file (adding a few lines that invoke two further ROM-BIOS service-functions), and then reassemble, relink, and reinstall your work

A valuable Online Reference

• Professor Ralf Brown’s 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)

Programming Details

• It’s easy to include ‘await keypress’:mov $0, %ah ; function-IDint $0x16 ; BIOS keyboard

service

• It’s easy to include ‘reboot system’:int $0x19 ; BIOS reboot service