Directives versus Instruction Mnemonics in .NET framework

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Directives versus Instruction Mnemonics
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Data definition directives look a little like machine instruction mnemonics, but they are emphatically not machine instructions One very common mistake made by beginners is looking for the binary opcode represented by a directive such as DB or DW There is no binary opcode for DW, DB, and the other directives Machine instructions, as the name implies, are instructions to the CPU itself Directives, by contrast, are instructions to the assembler Understanding directives is easier when you understand the nature of the assembler's job (Look back to 4 for a detailed refresher if you've gotten fuzzy on what assemblers and linkers do) The assembler scans your source code text file, and as it scans your source code file it builds an object code file on disk It builds this object code file step by step, one byte at a time, starting at the beginning of the file and working its way through to the end When it encounters a machine instruction mnemonic, it figures out what binary opcode is represented by that mnemonic and writes that binary opcode (which may be anywhere from one to six actual bytes) to the object code file When the assembler encounters a directive such as DW, it does not write any opcode to the object code file DW is a kind of signpost to the assembler, reading "Set aside two bytes of memory right here, for the value that follows" The DW directive specifies an initial value for the variable, and so the assembler writes the bytes corresponding to that value in the two bytes it set aside The assembler writes the address of the allocated space into a table, beside the label that names the variable Then the assembler moves on, to the next directive (if there are further directives) or to whatever comes next
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in the source code file For example, when you write the following statement in your assembly language program: MyVidOrg DW 0B800H
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what you are really doing is instructing the assembler to set aside two bytes of data (Define Word, remember) and place the value 0B800H in those two bytes The assembler writes the identifier MyVidOrg and the variable's address into a table it builds of identifiers (both labels and variables) in the program for later use by other elements of the program, or the linker
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The Difference between a Variable's Address and Its Contents
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I've left discussion of EATASM's machine instructions for last-at least in part because they're easy to explain All that EATASM does, really, is hand a string to DOS and tell DOS to display it on the screen by sending it to something called standard output It does this by passing the address of the string to DOS-not the character values contained in the string itself This is a crucial distinction that trips up a lot of beginners Here's the first instruction in EATASM: mov dx, eatmsg ; Mem data ref without [] loads the ADDRESS!
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If you look at the program, you can see that while DX is 2 bytes in size, the string eatmsg is 15 bytes in size At first glance, this MOV instruction would seem impossible-but that's because what's being moved is not the string itself, but the string's address, which (in the real mode flat model) is 16 bits-2 bytes-in size The address will thus fit nicely in DX When you place a variable's identifier in a MOV instruction, you are accessing the variable's address, as explained previously By contrast, if you want to work with the value stored in that variable, you must place the variable's identifier in square brackets Suppose you had defined a variable in the data section called MyData this way: MyData DW 0744H
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The identifier MyData represents some address in memory, and at that address the assembler places the value 0744H Now, if you want to copy the value contained in MyData to the AX register, you would use the following MOV instruction: MOV AX,[MyData] After this instruction, AX would contain 0744H There are many situations in which you need to move the address of a variable into a register rather than the contents of the variable In fact, you may find yourself moving the addresses of variables around more than the contents of the variables, especially if you make a lot of calls to DOS and BIOS services If you've used higher-level languages such as Basic and Pascal, this distinction may seem inane After all, who would mistake the contents of a variable for its location Well, that's easy for you to say-in Basic and Pascal you rarely if ever even think about where a variable is The language handles all that rigmarole for you In assembly language, knowing where a variable is located is essential in order to do lots of important things
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