IO Ports Hardwware Interrupts

Introduction Chapter 1 What is Assembly Language? Data Representation What is Assembly Language? A low-level processor-specific programming language design to match the processor’s machine instruction set each assembly language instruction matches exactly one machine language instruction we study here Intel’s 80x86 (and Pentiums) Why learn Assembly Language? To learn how high-level language code gets translated into machine language ie: to learn the details hidden in HLL code To learn the computer’s hardware by direct access to memory, video controller, sound card, keyboard… To speed up applications direct access to hardware (ex: writing directly to I/O ports instead of doing a system call) good ASM code is faster and smaller: rewrite in ASM the critical areas of code Assembly Language Applications Application programs are rarely written completely in assembly language only time-critical parts are written in ASM Ex: an interface subroutine (called from HLL programs) is written in ASM for direct hardware access Ex2: device drivers (called from the OS) ASM often used for embedded systems programs stored in PROM chips computer cartridge games, microcontrollers (automobiles, industrial plants...), telecommunication equipment… Very fast and compact but processor-specific Machine Language An assembler is a program that converts ASM code into machine language code: mov al,5 (Assembly Language) 1011000000000101 (Machine Language) most significant byte is the opcode for “move into register AL” the least significant byte is for the operand “5” Directly programming in machine language offers no advantage (over Assembly)... Binary Numbers are used to store both code and data On Intel’s x86: byte = 8 bits (smallest addressable unit) word = 2 bytes doubleword = 2 words quadword = 2 doublewords Number Systems A written number is meaningful only with respect to a base To tell the assembler which base we use: Hexadecimal 25 is written as 25h Octal 25 is written as 25o or 25q Binary 1010 is written as 1010b Decimal 1010 is written as 1010 or 1010d You are supposed to know how to convert from one base to another (see appendix A) Data Representation Even if we know that a block of memory contains data, to obtain its value we need to choose an interpretation Ex: memory content 0100 0001 can either represent: the number 2^{6} + 1 = 65 or the ASCII code of character “A” Signed and Unsigned Interpretation When a memory block contains a number, to obtain its value we must choose either: the signed interpretation: in that case the most significant bit (msb) represents the sign Positive number (or zero) if msb = 0 Negative number if msb = 1 the unsigned interpretation: in that case all the bits are used to represent a magnitude (ie: positive number, or zero) Twos Complement Notation Used to represent negative numbers The twos complement of a positive number X, denoted by NEG(X), is obtained by complementing all its bits and adding +1 NEG(X) = NOT(X) + 1 Ex: NEG(10) = NOT(10) + 1 = NOT(0000 1010b) + 1 = (1111 0101b) + 1 = 1111 0110b = NEG(10) = -10 It follows that X + NEG(X) = 0 To perform the difference X - Y: the machine executes the addition X + NEG(Y) Maximum and Minimum Values The msb of a signed number is used for its sign fewer bits are left for its magnitude Ex: for a signed byte smallest positive = 0000 0000b largest positive = 0111 1111b = 127 largest negative = -1 = 1111 1111b smallest negative = 1000 0000b = -128 Signed/Unsigned Interpretation (again) To obtain the value of a number we need to chose an interpretation Ex: memory content 1111 1111 can either represent: -1 if a signed interpretation is used 255 if an unsigned interpretation is used Only the programmer can provide an interpretation of the content of memory Character Representation Each character is represented by a 7-bit code: the ASCII code (from 00h to 7Fh) Only codes from 20h to 7Eh represent printable characters. The rest are control codes (used for printing, transmission…). An extended character set is obtained by setting the msb to 1 (codes 80h to FFh) so that each character is stored in 1 byte Varies from one system to another MS-DOS usage: for accentuated characters, Greek symbols and some graphic characters The ASCII character set CR = “carriage return” (MSDOS: move to beginning of line) LF = “line feed” (MSDOS: move directly one line below) SPC = “blank space” Text Files These are files containing only ASCII characters But different conventions are used for indicating an “end-of line” MS-DOS: + UNIX: MAC: This is at the origin of many problems encountered during transfers of text files from one system to another Strings and numbers A strings is stored as an array of characters A 1-byte ASCII code is stored for each char Hence, we can either store the number 123 in numerical form or as the string “123” The string form is best for display The numerical form is best for computations