Transcript Assembler
Week 6 Intro. to Assembly Language Dr. Muhammad Ayaz Levels of Programming Machine Language Assembly Language High level Language Computer Languages Applications High Level Language Low Level language Hardware 68000 Assembly Language C, JAVA Word, Excel HLL Complier Assembly Language Assembler Machine Language Machine Language (Low level) A computer can directly understand only its own machine language. Machine languages generally consist of streams of numbers (ultimately reduced to binary 1s and 0s). Machine-language programming proved Longer Codes Difficult to understand. Error prone (very easy to make mistakes). Very hard to find these mistakes Not Portable: must be rewritten on another computer architecture Machine Language Characteristics Direct memory management Little-to-no abstraction from the hardware Register access Statements usually have an obvious correspondence with clock cycles Superb performance Assembly Language To overcome the problems of speed and errors, assembly language is used. English-like abbreviations form the basis of assembly languages. Computers cannot understand assembly-language code until it is translated into machine language. Normally, designed for a family of processors. High level Language The main advantage of high-level languages over lowlevel language is that they are easier to read, write, and maintain. These are modern programming languages. C/C++, VB, JAVA, C#, Pascal, FORTRAN …. Much more simpler to understand and remember. High-level languages are closer to the human language. The early high-level programming languages were designed in the 1950s. Comparing Low & High-level Languages (1) Advantages of high-level language 1. 2. 3. 4. Advantages of HIGH level language more effective Reasons easier to learn, write, read and modify programs are shorter in length instructions are English-like format portable (can use in different computers with little modification) designed to solve problem one high-level language statement take the place of many machine instructions machine-independent Comparing Low & High-level Languages (2) Advantages of low-level language 1. 2. 3. Advantages of LOW level language more efficient Reasons hardware features, like bus width, is fully made use of require smaller memory space to fewer low-level instructions are run needed to do the same task (no redundant instructions produced by a translator) precise control of hardware some low-level instructions are designed specifically for controlling the hardware at low level 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 When to Use Assembly When speed and size matter ! Equipment that must response very quickly. Device driver. When the resource is limited. When we use the specialized instructions: 3D graphic library When there is no compiler ! When you want to understand internal architecture of a CPU ! Translation Process Assembler: translate Assembly to a Binary Code. check Syntax. produce an object file (not executable). Linker: combine one or more object files. resolve references to other object files / libraries. produce an executable program. Assembly Langu Translation Process (Cont’) Assembly Program Object File Assembler Object File Object File Natawut Nupairoj Libraries Linker Executable File Assembly Langu Assembly Language Statements Three types of statements in assembly language 1. 2. 3. Typically, one statement should appear on a line Executable Instructions Generate machine code for the processor to execute at runtime Instructions tell the processor what to do Assembler Directives Provide information to the assembler while translating a program Used to define data, select memory model, etc. Macros Shorthand notation for a group of statements Sequence of instructions, directives, or other macros 32-bit registers Upper (first) halves do not have names Registers The advantage of registers over computer memory is that they are extremely fast. The CPU has four general purpose programming registers, EAX, EBX, ECX and EDX. Each of the four general purpose programming registers is 32 bits wide Registers … Among these 32 bits, we can access the lower 16 bits of EAX (called AX) if we want to. Further, the lower 16 bits of EBX is called BX, etc. Moreover, we can access both the upper and lower 8 bits of AX (called AH and AL respectively), too, and similarly for BX, CX and DX. 16-bit Registers General-Purpose Registers These are Data Registers where the upper and lower half can be accessed separately AX = Accumulator (used in arithmetic) BX = Base (arithmetic, data movement…) CX = Counter (for looping instructions) DX = Data (multiplication & division) Index Registers SP (stack pointer) contains the offset of the top of the stack BP (base pointer) often contains the offset of a data/variable in the stack SI (source index) and DI (destination index) are used in string movement instructions SI points to the source string DI points to the destination string Segment Registers CS (code segment) holds the base location of all executable instructions in a program DS (data segment) the default base location for memory variables ES (extra segment) additional base location for memory variables SS (stack segment) base location for stack Status and Control Registers IP (instruction pointer) always contains the offset of the instruction to be executed next within the current code segment The FLAGS register consist of individual bits indicating either the mode of operation of the CPU (control flag) the outcome of an arithmetic operation (status)