X86 Assembly/Machine Language Conversion - Wikibooks, open books for an open world
mnemonic codes to refer to the different machine code instructions, instead What is the difference between the memory addresses of two consecutive machine. Assembly language is a deliberate, designed, abstraction of machine code. There is no requirement to have any specific relationship between. Relationship to Machine Code. X86 assembly instructions have a one-to- one relationship with the underlying machine instructions. This means that.
This approach was widely accepted in the early '80s the latter days of large-scale assembly language use. The language was classified as an assembler, because it worked with raw machine elements such as opcodesregistersand memory references; but it incorporated an expression syntax to indicate execution order.
Parentheses and other special symbols, along with block-oriented structured programming constructs, controlled the sequence of the generated instructions. A-natural was built as the object language of a C compiler, rather than for hand-coding, but its logical syntax won some fans. There has been little apparent demand for more sophisticated assemblers since the decline of large-scale assembly language development. REPEAT 20 switch rv nrandom, 9 ; generate a number between 0 and 8 mov ecx, 7 case 0 print "case 0" case ecx ; in contrast to most other programming languages, print "case 7" ; the Masm32 switch allows "variable cases" case They were once widely used for all sorts of programming.
However, by the s s on microcomputerstheir use had largely been supplanted by higher-level languages, in the search for improved programming productivity. Today assembly language is still used for direct hardware manipulation, access to specialized processor instructions, or to address critical performance issues. Typical uses are device driverslow-level embedded systemsand real-time systems. Historically, numerous programs have been written entirely in assembly language. Many commercial applications were written in assembly language as well, including a large amount of the IBM mainframe software written by large corporations.
Most early microcomputers relied on hand-coded assembly language, including most operating systems and large applications. This was because these systems had severe resource constraints, imposed idiosyncratic memory and display architectures, and provided limited, buggy system services. Perhaps more important was the lack of first-class high-level language compilers suitable for microcomputer use.
A psychological factor may have also played a role: In a more commercial context, the biggest reasons for using assembly language were minimal bloat sizeminimal overhead, greater speed, and reliability.
According to some[ who? This was in large part because interpreted BASIC dialects on these systems offered insufficient execution speed, as well as insufficient facilities to take full advantage of the available hardware on these systems. Some systems even have an integrated development environment IDE with highly advanced debugging and macro facilities. Some compilers available for the Radio Shack TRS and its successors had the capability to combine inline assembly source with high-level program statements.
Upon compilation a built-in assembler produced inline machine code. Current usage[ edit ] There have always been debates over the usefulness and performance of assembly language relative to high-level languages. Assembly language has specific niche uses where it is important; see below. In the case of speed optimization, modern optimizing compilers are claimed  to render high-level languages into code that can run as fast as hand-written assembly, despite the counter-examples that can be found.
This has made raw code execution speed a non-issue for many programmers. There are some situations in which developers might choose to use assembly language: A stand-alone executable of compact size is required that must execute without recourse to the run-time components or libraries associated with a high-level language; this is perhaps the most common situation.
For example, firmware for telephones, automobile fuel and ignition systems, air-conditioning control systems, security systems, and sensors. Code that must interact directly with the hardware, for example in device drivers and interrupt handlers. In an embedded processor or DSP, high-repetition interrupts require the shortest number of cycles per interrupt, such as an interrupt that occurs or times a second.
Programs that need to use processor-specific instructions not implemented in a compiler. A common example is the bitwise rotation instruction at the core of many encryption algorithms, as well as querying the parity of a byte or the 4-bit carry of an addition. Programs that create vectorized functions for programs in higher-level languages such as C. In the higher-level language this is sometimes aided by compiler intrinsic functions which map directly to SIMD mnemonics, but nevertheless result in a one-to-one assembly conversion specific for the given vector processor.
Programs requiring extreme optimization, for example an inner loop in a processor-intensive algorithm. Game programmers take advantage of the abilities of hardware features in systems, enabling games to run faster.
Also large scientific simulations require highly optimized algorithms, e. SIMD assembly version from x  Situations where no high-level language exists, on a new or specialized processor, for example. Programs that need precise timing such as real-time programs such as simulations, flight navigation systems, and medical equipment. For example, in a fly-by-wire system, telemetry must be interpreted and acted upon within strict time constraints.
Such systems must eliminate sources of unpredictable delays, which may be created by some interpreted languages, automatic garbage collectionpaging operations, or preemptive multitasking. However, some higher-level languages incorporate run-time components and operating system interfaces that can introduce such delays. Choosing assembly or lower level languages for such systems gives programmers greater visibility and control over processing details.
- Machine code
Modify and extend legacy code written for IBM mainframe computers. Computer virusesbootloaderscertain device driversor other items very close to the hardware or low-level operating system.
Instruction set simulators for monitoring, tracing and debugging where additional overhead is kept to a minimum Reverse-engineering and modifying program files such as existing binaries that may or may not have originally been written in a high-level language, for example when trying to recreate programs for which source code is not available or has been lost, or cracking copy protection of proprietary software.
Video games also termed ROM hackingwhich is possible via several methods. The most widely employed method is altering program code at the assembly language level. Self-modifying codeto which assembly language lends itself well. Games and other software for graphing calculators.
Although few programmers today regularly work with assembly language as a tool, the underlying concepts remain very important. Such fundamental topics as binary arithmeticmemory allocationstack processingcharacter set encoding, interrupt processing, and compiler design would be hard to study in detail without a grasp of how a computer operates at the hardware level.
Since a computer's behavior is fundamentally defined by its instruction set, the logical way to learn such concepts is to study an assembly language. Most modern computers have similar instruction sets. Therefore, studying a single assembly language is sufficient to learn: I the basic concepts; II to recognize situations where the use of assembly language might be appropriate; and III to see how efficient executable code can be created from high-level languages.
Typical applications[ edit ] Assembly language is typically used in a system's boot code, the low-level code that initializes and tests the system hardware prior to booting the operating system and is often stored in ROM.
Some compilers translate high-level languages into assembly first before fully compiling, allowing the assembly code to be viewed for debugging and optimization purposes. Some compilers for relatively low-level languages, such as Pascal or Callow the programmer to embed assembly language directly in the source code.
Programs using such facilities can then construct abstractions using different assembly language on each hardware platform.
X86 Assembly/Machine Language Conversion
The system's portable code can then use these processor-specific components through a uniform interface. Assembly language is useful in reverse engineering. Successor or derivative processor designs often include all the instructions of a predecessor and may add additional instructions.
Occasionally, a successor design will discontinue or alter the meaning of some instruction code typically because it is needed for new purposesaffecting code compatibility to some extent; even nearly completely compatible processors may show slightly different behavior for some instructions, but this is rarely a problem. Systems may also differ in other details, such as memory arrangement, operating systems, or peripheral devices.
Assembly language - Wikipedia
Because a program normally relies on such factors, different systems will typically not run the same machine code, even when the same type of processor is used. A processor's instruction set may have all instructions of the same length, or it may have variable-length instructions. How the patterns are organized varies strongly with the particular architecture and often also with the type of instruction.
Most instructions have one or more opcode fields which specifies the basic instruction type such as arithmetic, logical, jumpetc. An accumulator machine has a combined left operand and result in an implicit accumulator for most arithmetic instructions. Other architectures such as and the xfamily have accumulator versions of common instructions, with the accumulator regarded as one of the general registers by longer instructions.
A stack machine has most or all of its operands on an implicit stack. Special purpose instructions also often lack explicit operands CPUID in the x86 architecture writes values into four implicit destination registers, for instance.
This distinction between explicit and implicit operands is important in code generators, especially in the register allocation and live range tracking parts. A good code optimizer can track implicit as well as explicit operands which may allow more frequent constant propagationconstant folding of registers a register assigned the result of a constant expression freed up by replacing it by that constant and other code enhancements. Programs[ edit ] A computer program is a list of instructions that can be executed by a central processing unit.
A program's execution is done in order for the CPU that is executing it to solve a specific problem and thus accomplish a specific result.
While simple processors are able to execute instructions one after another, superscalar processors are capable of executing a variety of different instructions at once. Program flow may be influenced by special 'jump' instructions that transfer execution to an instruction other than the numerically following one. Conditional jumps are taken execution continues at another address or not execution continues at the next instruction depending on some condition.
Assembly language A much more readable rendition of machine language, called assembly languageuses mnemonic codes to refer to machine code instructions, rather than using the instructions' numeric values directly.