Thus far, you've seen a couple of HLA programs that will actually compile and run. However, all the statements appearing in programs to this point have been either data declarations or calls to HLA Standard Library routines. There hasn't been any real assembly language. Before we can progress any further and learn some real assembly language, a detour is necessary; unless you understand the basic structure of the Intel 80x86 CPU family, the machine instructions will make little sense.
The Intel CPU family is generally classified as a Von Neumann Architecture Machine. Von Neumann computer systems contain three main building blocks: the central processing unit (CPU), memory, and input/output (I/0) devices. These three components are interconnected using the system bus (consisting of the address, data, and control buses). The block diagram in Figure 1-4 shows this relationship.
The CPU communicates with memory and I/O devices by placing a numeric value on the address bus to select one of the memory locations or I/O device port locations, each of which has a unique binary numeric address. Then the CPU, memory, and I/O devices pass data among themselves by placing the data on the data bus. The control bus contains signals that determine the direction of the data transfer (to/from memory and to/from an I/O device).
The 80x86 CPU registers can be broken down into four categories: general-purpose registers, special-purpose application-accessible registers, segment registers, and special-purpose kernel-mode registers. Because the segment registers aren't used much in modern 32-bit operating systems (such as Windows, Mac OS X, FreeBSD, and Linux) and because this text is geared to writing programs written for 32-bit operating systems, there is little need to discuss the segment registers. The special-purpose kernel-mode registers are intended for writing operating systems, debuggers, and other system-level tools. Such software construction is well beyond the scope of this text.
The 80x86 (Intel family) CPUs provide several general-purpose registers for application use. These include eight 32-bit registers that have the following names: EAX, EBX, ECX, EDX, ESI, EDI, EBP, and ESP.
The E prefix on each name stands for extended. This prefix differentiates the 32-bit registers from the eight 16-bit registers that have the following names: AX, BX, CX, DX, SI, DI, BP, and SP.
Finally, the 80x86 CPUs provide eight 8-bit registers that have the following names: AL, AH, BL, BH, CL, CH, DL, and DH.
Unfortunately, these are not all separate registers. That is, the 80x86 does not provide 24 independent registers. Instead, the 80x86 overlays the 32-bit registers with the 16-bit registers, and it overlays the 16-bit registers with the 8-bit registers. Figure 1-5 shows this relationship.
The most important thing to note about the general-purpose registers is that they are not independent. Modifying one register may modify as many as three other registers. For example, modification of the EAX register may very well modify the AL, AH, and AX registers. This fact cannot be overemphasized here. A very common mistake in programs written by beginning assembly language programmers is register value corruption because the programmer did not completely understand the ramifications of the relationship shown in Figure 1-5.
The EFLAGS register is a 32-bit register that encapsulates several single-bit boolean (true/false) values. Most of the bits in the EFLAGS register are either reserved for kernel mode (operating system) functions or are of little interest to the application programmer. Eight of these bits (or flags) are of interest to application programmers writing assembly language programs. These are the overflow, direction, interrupt disable,[4] sign, zero, auxiliary carry, parity, and carry flags. Figure 1-6 shows the layout of the flags within the lower 16 bits of the EFLAGS register.
Of the eight flags that are of interest to application programmers, four flags in particular are extremely valuable: the overflow, carry, sign, and zero flags. Collectively, we will call these four flags the condition codes.[5] The state of these flags lets you test the result of previous computations. For example, after comparing two values, the condition code flags will tell you whether one value is less than, equal to, or greater than a second value.
One important fact that comes as a surprise to those just learning assembly language is that almost all calculations on the 80x86 CPU involve a register. For example, to add two variables together, storing the sum into a third variable, you must load one of the variables into a register, add the second operand to the value in the register, and then store the register away in the destination variable. Registers are a middleman in nearly every calculation. Therefore, registers are very important in 80x86 assembly language programs.
Another thing you should be aware of is that although the registers have the name "general purpose," you should not infer that you can use any register for any purpose. All the 80x86 registers have their own special purposes that limit their use in certain contexts. The SP/ESP register pair, for example, has a very special purpose that effectively prevents you from using it for anything else (it's the stack pointer). Likewise, the BP/EBP register has a special purpose that limits its usefulness as a general-purpose register. For the time being, you should avoid the use of the ESP and EBP registers for generic calculations; also, keep in mind that the remaining registers are not completely interchangeable in your programs.