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is allowed a high degree of portability for the time macros were used to customize large scale software systems for specific customers in the mainframe era and were also used by customer personnel to satisfy their employers needs by making specific versions of manufacturer operating systems this was done for example by systems programmers working with ibm s conversational monitor system virtual machine vm cms and with ibm s real time transaction processing add ons customer information control system cics and acp tpf the airline financial system that began in the 1970s and still runs many large computer reservation systems crs and credit card systems today it is also possible to use solely the macro processing abilities of an assembler to generate code written in completely different languages for example to generate a version of a program in cobol using a pure macro assembler program containing lines of cobol code inside assembly time operators instructing the assembler to generate arbitrary code ibm os 360 uses macros to perform system generation the user specifies options by coding a series of assembler macros assembling these macros generates a job stream to build the system including job control language and utility control statements this is because as was realized in the 1960s the concept of macro processing is independent of the concept of assembly the former being in modern terms more word processing text processing than generating object code the concept of macro processing appeared and appears in the c programming language which supports preprocessor instructions to set variables and make conditional tests on their values unlike certain previous macro processors inside assemblers the c preprocessor is not turing complete because it lacks the ability to either loop or go to the latter allowing programs to loop despite the power of macro processing it fell into disuse in many high level languages major exceptions being c c and pl i while remaining a perennial for assemblers macro parameter substitution is strictly by name at macro processing time the value of a parameter is textually substituted for its name the most famous class of bugs resulting was the use of a parameter that itself was an expression and not a simple name when the macro writer expected a name in the macro foo macro a load a b the intention was that the caller would provide the name of a variable and the global variable or constant b would be used to multiply a if foo is called with the parameter a c the macro expansion of load a c b occurs to avoid any possible ambiguity users of macro processors can parenthesize formal parameters inside macro definitions or callers can parenthesize the input parameters 30 support for structured programming edit packages of macros have been written providing structured programming elements to encode execution flow the earliest example of this approach was in the concept 14 macro set 31 originally proposed by harlan mills march 1970 and implemented by marvin kessler at ibm s federal systems division which provided if else endif and similar control flow blocks for os 360 assembler programs this was a way to reduce or eliminate the use of goto operations in assembly code one of the main factors causing spaghetti code in assembly language this approach was widely accepted in the early 1980s the latter days of large scale assembly language use ibm s high level assembler toolkit 32 includes such a macro package a curious design was a natural a stream oriented assembler for 8080 z80 processors citation needed from whitesmiths ltd developers of the unix like idris operating system and what was reported to be the first commercial c compiler the language was classified as an assembler because it worked with raw machine elements such as opcodes registers and 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 33 in spite of that they are still being developed and applied in cases where resource constraints or peculiarities in the target system s architecture prevent the effective use of higher level languages 34 assemblers with a strong macro engine allow structured programming via macros such as the switch macro provided with the masm32 package this code is a complete program include masm32 include masm32rt inc use the masm32 library code demomain 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 1 3 if eax 1 print case 1 elseif eax 2 print case 2 else print cases 1 to 3 other endif case 4 6 8 print cases 4 6 or 8 default mov ebx 19 print 20 stars repeat print dec ebx until si gn loop until the sign flag is set endsw print ch r 13 10 endm exit end demomain use of assembly language edit historical perspective edit assembly languages were not available at the time when the stored program computer was introduced kathleen booth is credited with inventing assembly language 35 36 based on theoretical work she began in 1947 while working on the arc2 at birkbeck university of london following consultation by andrew booth later her husband with mathematician john von neumann and physicist herman goldstine at the institute for advanced study 36 37 in late 1948 the electronic delay storage automatic calculator edsac had an assembler named initial orders integrated into its bootstrap program it used one letter mnemonics developed by david wheeler who is credited by the ieee computer society as the creator of the first assembler 19 38 39 reports on the edsac introduced the term assembly for the process of combining fields into an instruction word 40 soap symbolic optimal assembly program was an assembly language for the ibm 650 computer written by stan poley in 1955 41 assembly languages eliminate much of the error prone tedious and time consuming first generation programming needed with the earliest computers freeing programmers from tedium such as remembering numeric codes and calculating addresses they were once widely used for all sorts of programming however by the late 1950s citation needed their 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 42 typical uses are device drivers low level embedded systems and real time systems see current usage historically numerous programs have been written entirely in assembly language the burroughs mcp 1961 was the first computer for which an operating system was not developed entirely in assembly language it was written in executive systems problem oriented language espol an algol dialect many commercial applications were written in assembly language as well including a large amount of the ibm mainframe software written by large corporations cobol fortran and some pl i eventually displaced much of this work although a number of large organizations retained assembly language application infrastructures well into the 1990s 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 the first generation of microcomputer programmers retained a hobbyist wires and pliers attitude in a more commercial context the biggest reasons for using assembly language were minimal bloat size minimal overhead greater speed and reliability typical examples of large assembly language programs from this time are ibm pc dos operating systems the turbo pascal compiler and early applications such as the spreadsheet program lotus 1 2 3 assembly language was used to get the best performance out of the sega saturn a console that was notoriously challenging to develop and program games for 43 the 1993 arcade game nba jam is another example assembly language has long been the primary development language for many popular home computers of the 1980s and 1990s such as the msx sinclair zx spectrum commodore 64 commodore amiga and atari st 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 80 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 44 been debates over the usefulness and performance of assembly language relative to high level languages although assembly language has specific niche uses where it is important see below there are other tools for optimization 45 as of july 2017 update the tiobe index of programming language popularity ranks assembly language at 11 ahead of visual basic for example 46 assembler can be used to optimize for speed or optimize for size in the case of speed optimization modern optimizing compilers are claimed 47 to render high level languages into code that can run as fast as hand written assembly despite the counter examples that can be found 48 49 50 the complexity of modern processors and memory sub systems makes effective optimization increasingly difficult for compilers as well as for assembly programmers 51 52 moreover increasing processor performance has meant that most cpus sit idle most of the time 53 with delays caused by predictable bottlenecks such as cache misses i o operations and paging 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 writing code for systems with older processors clarification needed that have limited high level language options such as the atari 2600 commodore 64 and graphing calculators 54 programs for these computers of 1970s and 1980s are often written in the context of demoscene or retrogaming subcultures 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 1000 or 10000 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 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 examples have included firmware for telephones automobile fuel and ignition systems air conditioning control systems security systems and sensors programs with performance sensitive inner loops where assembly language provides optimization opportunities that are difficult to achieve in a high level language for example linear algebra with blas 48 55 or discrete cosine transformation e g simd assembly version from x264 56 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 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 collection paging 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 cryptographic algorithms that must always take strictly the same time to execute preventing timing attacks modify and extend legacy code written for ibm mainframe computers 57 58 situations where complete control over the environment is required in extremely high security situations where nothing can be taken for granted computer viruses bootloaders certain device drivers or 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 situations where no high level language exists on a new or specialized processor for which no cross compiler is available 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 hacking which is possible via several methods the most widely employed method is altering program code at the assembly language level assembly language is still taught in most computer science and electronic engineering programs although few programmers today regularly work with assembly language as a tool the underlying concepts remain important such fundamental topics as binary arithmetic memory allocation stack processing character 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 b...