The Bones of an Assembly Language Program in .NET framework

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The Bones of an Assembly Language Program
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The following listing is perhaps the simplest correct program that will do anything visible and still be comprehensible and expandable This issue of comprehensibility is utterly central to quality assembly language programming With no other computer language (not even APL or that old devil FORTH) is there anything even close to the risk of writing code that looks so much like something scraped off the wall of King Tut's tomb The program EATASM displays one (short) line of text on your display screen: Eat at Joe's! For that you have to feed 28 lines of text file to the assembler Many of those 28 lines are unnecessary in the strict sense, but serve instead as commentary to allow you to understand what the program is doing (or more important, how it's doing it) six months or a year from now One of the aims of assembly language coding is to use as few instructions as possible in getting the job done This does not mean creating as short a source code file as possible (The size of the source file has nothing to do with the size of the executable file assembled from it!) The more comments you put in your file, the better you'll remember how things work inside the program the next time you pick it up I think you'll find it amazing how quickly the logic of a complicated assembly language file goes cold in your head After no more than 48 hours of working on other projects, I've come back to assembler projects and had to struggle to get back to flank speed on development Comments are neither time nor space wasted IBM used to say, One line of comments per line of code That's good-and should be considered a minimum for assembly language work A better course (that I will in fact follow in the more complicated examples later on) is to use one short line of commentary to the right of each line of code, along with a comment block at the start of each sequence of instructions that work together in accomplishing some discrete task Here's the program Read it carefully: ; ; ; ; ; ; ; ; ; Source name Executable name Code model: Version Created date Last update Author Description : : : : : : : : EATASM EATCOM Real mode flat model 10 6/4/1999 9/10/1999 Jeff Duntemann A simple example of a DOS COM file programmed using NASM-IDE 11 and NASM 098 ; Set 16 bit code generation ; Set code start address to 100h (COM file) ; Section containing code ; Mem data ref without [] loads the ADDRESS! ; Function 9 displays text to standard output ; INT 21H makes the call into DOS ; This DOS function exits the program ; and returns control to DOS ; Section containing initialized data
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[BITS 16] [ORG 0100H] [SECTION text] START: mov mov int mov int dx, eatmsg ah,9 21H ax, 04C00H 21H
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[SECTION data] eatmsg
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db "Eat at Joe's!", 13, 10, "$" ;Here's our message
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After all our discussion in previous chapters about segments, this program might seem, um, suspiciously simple And indeed it's simple, and it's simple almost entirely because it's written for the 16-bit real mode flat model (I drew this model out in Figure 68) The first thing you'll notice is that there are no references to segments or segment registers anywhere The reason for this is that in real mode flat model, you are inside a single segment, and everything you do, you do within that single segment If everything happens within one single segment, the segments (in a sense) "factor out" and you can imagine that they don't exist Once we assemble EATASM and create a runnable program from it, I'll show you what those segment registers are up to and how it is that you can almost ignore them in real mode flat model But first, let's talk about what all those lines are doing At the top is a summary comment block This text is for your use only When NASM processes a ASM file, it strips out and discards all text between any semicolon and the end of the line the semicolon is in Such lines are comments, and they serve only to explain what's going on in your program They add nothing to the executable file, and they don't pass information to the assembler I recommend placing a summary comment block like this at the top of every source code file you create Fill it with information that will help someone else understand the file you've written or that will help you understand the file later on, after it's gone cold in your mind Beneath the comment block is a short sequence of commands directed to the assembler These commands are placed in square brackets so that NASM knows that they are for its use, and are not to be interpreted as part of the program The first of these commands is this: [BITS 16] ; Set 16 bit code generation
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The BITS command tells NASM that the program it's assembling is intended to be run in real mode, which is a 16-bit mode Using [BITS 32] instead would have brought into play all the marvelous 32-bit protected mode goodies introduced with the 386 and later x86 CPUs On the other hand, DOS can't run protected mode programs, so that wouldn't be especially useful The next command requires a little more explanation: [ORG 0100h] ; Set code start address to 100h (COM file)
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"ORG" is an abbreviation of origin, and what it specifies is sometimes called the origin of the program, which is where code execution begins Code execution begins at 0100H for this program The 0100h value (the h and H are interchangeable) is loaded into the instruction pointer IP by DOS when the program is loaded and run So, when DOS turns control over to your program (scary thought, that!), the first instruction to be executed is the one pointed to by IP-in this case, at 0100H Why 0100H Look back at Figure 68 The real mode flat model (which is often called the COM file model) has a 256-byte prefix at the beginning of its single segment This is the Program Segment Prefix (PSP) and it has several uses that I won't be explaining here The PSP is basically a data buffer and contains no code The code cannot begin until after the PSP, so the 0100H value is there to tell DOS to skip those first 256 bytes The next command is this: [SECTION text] ; Section containing code
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NASM divides your programs into what it calls sections These sections are less important in real mode flat model than in real mode segmented model, when sections map onto segments (More on this later) In flat model, you have only one segment But the SECTION commands tell NASM where to look for particular types of things In the text section, NASM expects to find program code A little further down the file you'll see another SECTION command, this one for the data section In the data section, NASM expects to find the definitions for your initialized variables A third section is possible, the bss section, which contains uninitialized data EATASM does not use any uninitialized data, so this section does not exist in this program I discuss uninitialized data later on, in connection with the stack
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