MACHINE CONTROL AND MISCELLANEOUS INSTRUCTIONS
The last category of real mode instructions found in the microprocessor is the machine control and miscellaneous group. These instructions provide control of the carry bit, sample the BUSY/TEST pin, and perform various other functions. Because many of these instructions are used in hardware control, they need only be explained briefly at this point.
Controlling the Carry Flag Bit
The carry flag (C) propagates the carry or borrow in multiple-word/doubleword addition and sub- traction. It also can indicate errors in assembly language procedures. Three instructions control the contents of the carry flag: STC (set carry), CLC (clear carry), and CMC (complement carry).
Because the carry flag is seldom used except with multiple-word addition and subtraction, it is available for other uses. The most common task for the carry flag is to indicate an error upon return from a procedure. Suppose that a procedure reads data from a disk memory file. This operation can be successful, or an error such as file-not-found can occur. Upon return from this procedure, if C = 1, an error has occurred; if C = 0, no error occurred. Most of the DOS and BIOS procedures use the carry flag to indicate error conditions. This flag is not available in Visual C/C++ for use with C++.
WAIT
The WAIT instruction monitors the hardware BUSY pin on the 80286 and 80386, and the TEST pin on the 8086/8088. The name of this pin was changed beginning with the 80286 microprocessor from TEST to BUSY. If the WAIT instruction executes while the BUSY pin = 1, nothing hap- pens and the next instruction executes. If the BUSY pin = 0 when the WAIT instruction executes, the microprocessor waits for the BUSY pin to return to a logic 1. This pin inputs a busy condition when at a logic 0 level.
The BUSY / TEST pin of the microprocessor is usually connected to the BUSY pin of the 8087 through the 80387 numeric coprocessors. This connection allows the microprocessor to wait until the coprocessor finishes a task. Because the coprocessor is inside an 80486 through the Core2, the BUSY pin is not present in these microprocessors.
HLT
The halt instruction (HLT) stops the execution of software. There are three ways to exit a halt: by an interrupt, by a hardware reset, or during a DMA operation. This instruction normally appears in a program to wait for an interrupt. It often synchronizes external hardware interrupts with the software system. Note that DOS and Windows both use interrupts extensively, so HLT will not halt the computer when operated under these operating systems.
NOP
When the microprocessor encounters a no operation instruction (NOP), it takes a short time to execute. In early years, before software development tools were available, a NOP, which per- forms absolutely no operation, was often used to pad software with space for future machine lan- guage instructions. If you are developing machine language programs, which are extremely rare, it is recommended that you place 10 or so NOPS in your program at 50-byte intervals. This is done in case you need to add instructions at some future point. A NOP may also find application in time delays to waste time. Realize that a NOP used for timing is not very accurate because of the cache and pipelines in modem microprocessors.
LOCK Prefix
The LOCK prefix appends an instruction and causes the LOCK pin to become a logic 0. The LOCK pin often disables external bus masters or other system components. The LOCK prefix causes the LOCK pin to activate for only the duration of a locked instruction. If more than one sequential instruction is locked, the LOCK pin remains a logic 0 for the duration of the sequence of locked instructions. The LOCK:MOV AL,[SI] instruction is an example of a locked instruction.
ESC
The escape (ESC) instruction passes instructions to the floating-point coprocessor from the microprocessor. Whenever an ESC instruction executes, the microprocessor provides the memory address, if required, but otherwise performs a NOP. Six bits of the ESC instruction provide the opcode to the coprocessor and begin executing a coprocessor instruction.
The ESC opcode never appears in a program as ESC and in itself is considered obsolete as an opcode. In its place are a set of coprocessor instructions (FLD, FST, FMUL, etc.) that assemble as ESC instructions for the coprocessor. More detail is provided in Chapter 13, which details the 8087–Core2 numeric coprocessors.
BOUND
The BOUND instruction, first made available in the 80186 microprocessor, is a comparison instruction that may cause an interrupt (vector type number 5). This instruction compares the contents of any 16-bit or 32-bit register against the contents of two words or doublewords of memory: an upper and a lower boundary. If the value in the register compared with memory is not within the upper and lower boundary, a type 5 interrupt ensues. If it is within the boundary, the next instruction in the program executes.
For example, if the BOUND SI,DATA instruction executes, word-sized location DATA contains the lower boundary, and word-sized location DATA+2 bytes contains the upper boundary. If the number contained in SI is less than memory location DATA or greater than memory location DATA+2 bytes, a type 5 interrupt occurs. Note that when this interrupt occurs, the return address points to the BOUND instruction, not to the instruction following BOUND. This differs from a normal interrupt, where the return address points to the next instruction in the program.
ENTER and LEAVE
The ENTER and LEAVE instructions, first made available to the 80186 microprocessor, are used with stack frames, which are mechanisms used to pass parameters to a procedure through the stack memory. The stack frame also holds local memory variables for the procedure. Stack frames provide dynamic areas of memory for procedures in multiuser environments.
The ENTER instruction creates a stack frame by pushing BP onto the stack and then loading BP with the uppermost address of the stack frame. This allows stack frame variables to be accessed through the BP register. The ENTER instruction contains two operands: The first operand specifies the number of bytes to reserve for variables on the stack frame, and the second specifies the level of the procedure.
Suppose that an ENTER 8,0 instruction executes. This instruction reserves 8 bytes of memory for the stack frame and the zero specifies level 0. Figure 6–10 shows the stack frame set up by this instruction. Note that this instruction stores BP onto the top of the stack. It then sub- tracts 8 from the stack pointer, leaving 8 bytes of memory space for temporary data storage. The uppermost location of this 8-byte temporary storage area is addressed by BP. The LEAVE instruction reverses this process by reloading both SP and BP with their prior values. The ENTER and LEAVE instructions were used to call C++ functions in Windows 3.1, but since then, CALL has been used in modern versions of Windows for C++ functions.
FIGURE 6–10 The stack frame created by the ENTER 8,0 instruction. Notice that BP is stored beginning at the top of the stack frame. This is followed by an 8-byte area called a stack frame.
