CS 630: Advanced Microcomputer Programming

Spring 2004

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 AMD, Cyrix, clones

• Even present as ‘legacy mode’ in AMD64

• For study purposes we can pretend we’re studying a ‘bare machine’ (i.e., no OS)

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 program code

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 Diagram

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

Review of memory addressing

• Implicit addressing (e.g. push eax, scasb, xlat, …)

• Direct addressing(e.g., inc salary, mov counter,#0, …)

• Indirect addressing(e.g., add [ebx],cl , pop word [bx+si]

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 2:30pm-4:00pm

• Phone: (415) 422-6562

• Email: cruse@usfca.edu

• Webpage: nexus.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)

Recent 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)• 2000: Pentium 4 (new SIMD instructions)

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 this 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 Enable) 0=no, 1=yes

SMSW AXOR AX, #1LMSW 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 programs 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 system provides ‘as86’)

• This assembler’s syntax is similar to the standard set by Intel and Microsoft, but it differs from the AT&T-style syntax that is used with the Linux ‘as’ assembler

• Syntax is documented online: $ man as86

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)

Compiling and Installing

• Compiling our ‘boot loader’ using as86 is a one-step operation:

$ as86 bootload.s –b bootload.b

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

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

Executing a ‘boot-loader’

• Perform a system reset (CTRL-ALT-DEL)

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

• GRUB will display a menu of Boot Options

• You can choose to boot from floppy disk

• Another option: boot from a diskette-image

In-Class Exercises

• Go to our class website:http://nexus.cs.usfca.edu/~cruse/cs630

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

• Reboot machine and use GRUB’s menu to boot our demo from the floppy diskette

• Modify our demo so it will ‘reboot’ (instead of freeze) when a user presses any key

Programming Details

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

service

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

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)