MULTI PASS ASSEMBLERINTRODUCTION Multi pass assembler means more than one pass is used by assembler. Multi pass assembler is used to eliminate forward references in symbol definition. It creates a number of passes that is necessary to process the definition of symbols. Multi pass assembler does the work in two pass by resolving the forward references. First pass Scans the code Validates the tokens Creates a symbol table Second Pass Solves forward references Converts the code to the machine IMPLEMENTATION A programming language that is one step away from machine language. Each assembly language statement is translated into one machine instruction by the assembler. Programmers must be well versed in the computer's architecture, and, undocumented assembly language programs are difficult to maintain. It is hardware dependent; there is a different assembly language for each CPU series. Pass 1 Assign addresses to all statements in the program Save the values assigned to all labels for use in Pass 2 1 their lengths and their op-codes (or subroutine to use for translation) Symbol table (ST): contains labels and their values String storage buffer (SSB): contains ASCII characters for the strings Forward references table (FRT): contains pointer to the string in SSB and offset where its value will be inserted in the object code 2 . WORD Perform processing of assembler directives not done in Pass 1 MULTI PASS ASSEMBLER OVER SOURCE CODE A two-pass assembler performs two sequential scans over the source code: Pass 1: symbols and literals are defined Pass 2: object program is generated Parsing: moving in program lines to pull out op-codes and operands DATA STRUCTURES Location counter (LC): points to the next location where the code will be placed Op-code translation table: contains symbolic instructions. Perform some processing of assembler directives Pass 2 Assemble instructions Generate data values defined by BYTE. but would not be able to determine the address of the forward reference FWD when assembling the branch statement S1. no earlier than the point where the symbol is defined) telling the linker or the loader to "go back" and overwrite a placeholder which had been left where the as yet undefined symbol was used. indeed FWD may be undefined. One-pass assemblers go through the source code once. EXAMPLE In the following code snippet a one-pass assembler would be able to determine the address of the backward reference BKWD when assembling statement S2. then use the table in later passes to generate code... Any symbol used before it is defined will require "errata" at the end of the object code (or. A simple two pass assembler NUMBER OF PASSES COMPARISON There are two types of assemblers based on how many passes through the source are needed (how many times the assembler reads the source) to produce the executable program.. FWD EQU * . at least. 3 . A two-pass assembler would determine both addresses in pass 1. so they would be known when generating code in pass 2. Multi-pass assemblers create a table with all symbols and their values in the first passes.. S1 B FWD . . allow the programmer to embed assembly language directly in the source code. BKWD EQU * . can then construct abstractions using different assembly 4 . The advantage of the multi-pass assembler is that the absence of errata makes the linking process (or the program load if the assembler directly produces executable code) faster. the low-level code that initializes and tests the system hardware prior to booting the operating system and is often stored in ROM. APPLICATIONS Assembly language is typically used in a system's boot code. Relatively low-level languages. In an assembler with peephole optimization.) Some compilers translate high-level languages into assembly first before fully compiling. such as C. With modern computers this has ceased to be an issue. such as the Linux kernel. (BIOS on IBM-compatible PC systems and CP/M is an example. addresses may be recalculated between passes to allow replacing pessimistic code with code tailored to the exact distance from the target. the assembler will make a pessimistic estimate when first encountering the operation. This means that if the size of an operation referring to an operand defined later depends on the type or distance of the operand. Programs using such facilities. allowing the assembly code to be viewed for debugging and optimization purposes.. the assembler must be able to determine the size of each instruction on the initial passes in order to calculate the addresses of subsequent symbols. S2 B BKWD In both cases. The original reason for the use of one-pass assemblers was speed of assembly – often a second pass would require rewinding and rereading the program source on tape or rereading a deck of cards or punched paper tape. and if necessary pad it with one or more "no-operation" instructions in a later pass or the errata. club. REFERENCES http://www.classle.cs.blogspot.co. with each line of input. this approach results in two very similar procedures which must be maintained in parallel.quora. Assembly language is used to enhance speed of execution. but it adds to the input/output burden of the system.in/books?id=IUVm4y- XRecC&pg=PA103&lpg=PA103&dq=multi+pass+assembler&source=bl&ots=7B ry8tSOl6&sig=7Uk1cnIWD7- oTXr_Ad9_o1ABhfc&hl=en&sa=X&ved=0ahUKEwj9nIK8qIzRAhUMuI8KHbc HDHA4ChDoAQguMAU#v=onepage&q=multi%20pass%20assembler&f=false solomon. the positions of some or all of the lexemes on that line and some of the results of parsing. On the other hand.edu/~jones/syssoft/fall00/notes/04fwd.uiowa. For example. whether the line contained a label and what the value of the location counter was prior to processing that line.com/site/assignmentssolved/mca/semester3/mc0073/30 https://www.html https://www. and which occupy almost twice the memory space occupied.in/2013/04/what-is-multi-pass-assembler. especially in early Personal Computers with limited processing power and RAM. in the example assembly language. since no flag must be tested to determine what to evaluate.html https://sites.com/Q/What_is_multi_pass_assembler http://albertstu16.tw/Course/SP. The advantage of writing assemblers with a separate procedure for each pass is that this leads to assemblers which run very fast.net/#!/classle/book/system-software-and-assemblers-2-marks 5 . each line might be sent from one pass to the next with a record indicating whether the line was a definition. language on each hardware platform. or a statement.answers.ipv6. CONCLUSION Sometimes. for statements. and.com/What-are-advantages-of-assembler-with-multiple-passes https://books. The use of such an intermediate file can almost completely eliminate redundant computations in the two passes.google.pdf http://homepage.google. The system's portable code can then use these processor-specific components through a uniform interface.941/sp2-4. The advantage of this is that the first pass can record. the first pass of an extreme two-pass assembler produces an output file which is then read in by the second pass.