Chapter – 2 The Microprocessor & its Architecture









This chapter presents the microprocessor as a programmable device by first looking at its in­ternal programming model and then at how it addresses its memory space. The addressing modes for Intel family of microprocessors are described for both the real and protected modes of operation. Real mode memory exists at locations 00000H-FFFFFH the first 1M byte of the memory system—and is present on all versions of the microprocessor. Pro­tected mode memory exists at any location in the entire memory system, but is available only to the 80286—Pentium II, not to the earlier 8086 or 8088 microprocessors. Protected mode memory for the 80286 contains 16M bytes; for the 80386-Pentium, 4G bytes; and for the Pentium Pro and Pen­tium II, either 4G or 64G bytes.


2.1.     Internal Microprocessor Architecture


Before a program is written or any instruction investigated, the internal configuration of the mi­croprocessor must be known. This section of the chapter details the program-visible internal ar­chitecture of the 8086—80486 and the Pentium—Pentium II microprocessors. Also detailed are the function and purpose of each of these internal registers.


2.1.1.  The Programming Model


         The programming model of the 8086 through the Pentium II’s considered to be program visible because its registers are used during application programming and are specified by the instruc­tions. Other registers, detailed later in this chapter, are considered to be program invisible be­cause they are not addressable directly during applications programming, but may be used indirectly during system programming. Only the 80286 and above contain the program-invisible registers used to control and operate the protected memory system.

Figure 2-1 illustrates the programming model of the 8086 through the Pentium II micro­processor. The earlier 8086, 8088, and 80286 contain 16-bit internal architectures, a subset of the registers. The 80386, 80486, Pentium, Pentium Pro, and Pentium II mi­croprocessors contain full 32-bit internal architectures. The architectures of the earlier 8086 through the 80286 are fully upward-compatible to the 80386 through the Pentium II. The shaded areas in this illustration represent registers that are not found in the 8086, 8088, or 80286 micro­processors.





















       The programming model contains 8-, 16-, and 32-bit registers. The 8-bit registers are AH, AL, BH, BL, CH, CL, DH, and DL and are referred to when an instruction is formed using these two-letter designations. The 16-bit registers are AX, BX, CX, DX, SP, BP, DI, SI, IP, FLAGS, CS, DS, ES, SS, FS, and GS. The extended 32-bit registers are EAX, EBX, ECX, EDX, ESP, EBP, EDI, ESI, EIP, and EFLAGS. These 32-bit extended registers, and 16-bit registers ES and GS are available only in the 80386 and above.

Some registers are general-purpose or multipurpose registers, while some have special purposes. The multipurpose registers include EAX, EBX, ECX, EDX, EBP, EDI, and ESI. These registers hold various data sizes (bytes, words, or doublewords) and are used for almost any purpose, as dictated by a program.                          Multipurpose Registers


EAX (accumulator)

EAX is referenced as a 32-bit register (EAX), as a 16-bit register (AX), or as either of two 8-bit registers (AH and AL). Note that if an 8- or 16-bit register is addressed, only that portion of the 32-bit register changes without affecting the remaining bits. The accumu­lator is used for instructions such as multiplication, division, and some of the adjustment instructions. For these instructions, the accumulator has a special purpose, but is generally considered to be a multipurpose register. In the 80386 and above, the EAX register may also hold the offset address of a location in the memory system.


EBX (base index)

EBX is addressable as EBX, BX, BH, or BL. The BX register sometimes holds the offset address of a location in the memory system in all versions of the microprocessor. In the 80386 and above, EBX also can address memory data.


ECX (count)

ECX is a general-purpose register that also holds the count for various instructions. In the 80386 and above, the ECX register also can hold the offset address of memory data. Instructions that use a count are the repeated string instructions (REP/REPE/REPNE); and shift, rotate, and LOOP/LOOPD instructions. The shift and rotate instructions use CL as the count, the repeated string instructions use CX, and the LOOP/LOOPD instructions use either CX or ECX.


EDX (data)

EDX is a general-purpose register that holds a part of the result from a multiplication or part of the dividend before a division. In the 80386 and above, this register can also address memory data.


EBP (base pointer)

EBP points to a memory location in all versions of the micropro­cessor for memory data transfers. This register is addressed as either BP or EBP.


EDI (destination index)

EDI often addresses string destination data for the string instruc­tions. It also functions as either a 32-bit (EDI) or 16-bit (DI) general-purpose register.


ESI (source index)

ESI is used as either ESI or SI. The source index register often addresses source string data for the string instructions. Like EDI, ESI also functions as a general-purpose register. As a 16-bit register, it is addressed as SI; as a 32-bit register, it is addressed as ESI.                          Special-purpose Registers.


The special-purpose registers include EIP, ESP, EFLAGS; and the segment registers CS, DS, ES, SS, FS, and GS.


EIP (instruction pointer)

EIP addresses the next instruction in a section of memory defined as a code segment. This register is IP (16 bits) when the micropro­cessor operates in the real mode and EIP (32 bits) when the 80386 and above operate in the protected mode. Note that the 8086, 8088, and 80286 do contain EIP, and only the 80286 and above operate in the protected mode. The instruction pointer, which points to the next instruction in a program, is used by the microprocessor to find the next sequential instruction in a program located within the code segment. The instruction pointer can be modified with a jump or a call instruction.


ESP (stack pointer)

ESP addresses an area of memory called the stack. The stack memory stores data through this pointer. This register is referred to as SP if used as a 16-hit register and ESP if referred to as a 32-bit register.







EFLAGS indicate the condition of the microprocessor and control its operation. Figure 2-2 shows the flag registers of all versions of the microprocessor. Note that the flags are upward-compatible from the 8086/8088 to the Pentium II microprocessor. The 8086-80286 contain a FLAG register (16 bits) and the 80386 and above contain an EFLAG register (32-bit extended flag register).



The rightmost five flag bits and the overflow flag change after many arithmetic and logic in­structions execute. The flags never change for any data transfer or program control operation. Some of the flags are also used to control features found in the microprocessor. Following is a list of each flag bit, with a brief description of their function.


C (carry)

Carry holds the carry after addition or the borrow after subtraction. The carry flag also indicates error conditions, as dictated by some programs and procedures. This is especially true of the DOS func­tion calls.


P (parity)

Parity is a logic 0 for odd parity and a logic 1 for even parity. Parity is a count of ones in a number expressed as even or odd.

If a number contains zero one bits, it has even parity. The parity flag finds little application in modern programming and was implemented in early Intel microprocessors for checking data in data communications environments. Today parity checking is often accomplished by the data communications equipment instead of the microprocessor.


A (auxiliary carry)

The auxiliary carry holds the carry (half-carry) after addition or the borrow after subtraction between bits positions 3 and 4 of the result. This highly specialized flag bit is tested by the DAA and DAS instructions to adjust the value of AL after a BCD addition or subtraction. Otherwise, the A flag bit is not used by the microprocessor or any other instructions.


Z (zero)

The zero flag shows that the result of an arithmetic or logic opera­tion is zero. If Z=1, the result is zero; if Z= 0, the result is not zero.


S (sign)

The sign flag holds the arithmetic sign of the result after an arithmetic or logic instruction executes. If S=1, the sign bit (leftmost hit of a number) is set or negative; if S=0, the sign bit is cleared or positive.


T (trap)

The trap flag enables trapping through an on-chip debugging feature. (A program is debugged to find an error or bug.) If the T flag is enabled (1), the microprocessor interrupts the flow of the program on conditions as indicated by the debug registers and control registers. lf the T flag is a logic 0, the trapping (debugging) feature is disabled.


I (interrupt)

The interrupt flag controls the operation of the INTR (interrupt request) input pin. If I=1. the INTR pin is enabled: if I= 0, the INTR pin is disabled. The state of the I flag bit is controlled by the STI (set I flag) and CLI (clear I flag) instructions.


D (direction)

The direction flag selects either the increment or decrement mode for the Dl and/or SI registers during string instructions. If D=1, the registers are automatically decremented: if D=1, the registers are automatically incremented. The D flag is set with the STD (set direction) and cleared with the CLD (clear direction) instructions.


0 (overflow)

Overflows occurs when signed numbers are added or subtracted. An overflow indicates that the result has exceeded the capacity of the machine. For unsigned operations, the overflow flag is ignored.


IOPL (I/0 privilege level)

IOPL is used in protected mode operation to select the privilege level for I/O devices. If the current privilege level is higher or more trusted than the IOPL, I/O executes without hindrance. If the IOPL is lower than the current privilege level, an interrupt occurs, causing execution to suspend. Note that an IOPL of 00 is the highest or most trusted: if IOPL is 11,   it is the lowest or least trusted.


NT (nested task)

The nested task flag indicates that the current task is nested within another task in protected mode operation. This line is set when the task is nested by software.


RF (resume)

The resume flag is used with debugging to control the resumption of execution after the next instruction.


VM (virtual mode)

The VM flag bit selects virtual mode operation in a protected mode system. A virtual mode system allows multiple DOS memory par­titions that are 1M byte in length to coexist in the memory system. Essentially, this allows the system program to execute multiple DOS programs.


AC (alignment check)

The alignment check flag bit activates if a word or douhleword is addressed on a non-word or non-douhleword boundary. Only the 80486SX microprocessor contains the alignment check hit that is primarily used by its companion numeric coprocessor, the 80487SX, for synchronization.


VIF (virtual interrupt flag)

The VIF is a copy of the interrupt flag bit available to the Pentium-Pentium II microprocessors.


VIP (virtual interrupt pending)

VIP provides information about a virtual mode interrupt for the Pentium—Pentium II microprocessors. This is used in multitasking environments to provide the operating system with virtual interrupt flags and interrupt pending information.


ID (identification)

The ID flag indicates that the Pentium—Pentium II microprocessors support the CPUID instruction. The CPUID instruction provides the system with information about the Pentium microprocessor, such as its version number and manufacturer.             Segment Registers


Additional registers, called segment registers, generate memory addresses when combined with other registers in the microprocessor. There are either four or six segment registers in various versions of the microprocessor. A segment register functions differently in the real mode when compared to the protected mode operation of the microprocessor. Following is a list of each segment register, along with its function in the system:


CS (code)

The code segment is a section of memory that holds the code (programs and procedures) used by the microprocessor. The code segment register defines the starting address of the section of memory holding code. In real mode operation, it defines the start of a 64K-byte section of memory; in protected mode, it selects a descriptor that describes the starting address and length of a section of memory holding code. The code segment is limited to 64K bytes in the 8088-80286, and 4G bytes in the 80386 and above when these microprocessors operate in the protected mode.


DS (data)

The data segment is a section of memory that contains most data used by a program. Data are accessed in the data segment by an offset address or the contents of other registers that hold the offset address. As with the code segment and other segments, the length is limited to 64K bytes in the 8086-80286, and 4G bytes in the 80386 and above.


ES (extra)

The extra segment is an additional data segment that is used by some of the string instructions to hold destination data.


SS (stack)

The stack segment defines the area of memory used for the stack. The stack entry point is determined by the stack segment and stack pointer registers. The BP register also addresses data within the stack segment.


FS and GS

The FS and GS segments are supplemental segment registers available in the 80386, 80486, Pentium. and Pentium Pro microprocessors to allow two additional memory segments for access by programs.


2.2.     Real Mode Memory Addressing


         The 80286 and above operate in either the real or protected mode. Only the 8086 and 8088 op­erate exclusively in the real mode. Real mode operation allows the microprocessor to address only the first 1M byte of memory space-even if it is the Pentium II microprocessor. Note that the first 1 M byte of memory is called either the real memory or conventional memory system. The DOS operating system requires the microprocessor to operate in the real mode. Real mode oper­ation allows application software written for the 8086/8088, which contain only 1 M byte of memory, to function in the 80286 and above without changing the software. The upward com­patibility of software is partially responsible for the continuing success of the Intel family of mi­croprocessors. In all cases, each of these microprocessors begins operation in the real mode by default whenever power is applied or the microprocessor is reset.


2.2.1.  Segments And Offsets


         A combination of a segment address and an offset address, access a memory location in the real mode. All real mode memory addresses must consist of a segment address plus an offset address. The segment address, located within one of the segment registers, defines the beginning address of any 64K-byte memory segment. The offset address selects any location within the 64K byte memory segment. Segments in the real mode always have a length of 64K bytes. Figure 2-3 shows how the segment plus offset addressing scheme selects a memory location. This illustra­tion shows a memory segment that begins at location 1 0000H and ends at location 1 FFFEH 64K bytes in length. It also shows how an offset address, sometimes called a displacement, of F000H selects location             1F000H in the memory system. Note that the offset or displacement is the distance above the start of the segment, as shown in Figure 2-3.

       The segment register in Figure 2-3 contains a 1000H, yet it addresses a starting segment at location 10000H. In the real mode, each segment register is internally appended with a 0H on its rightmost end. This forms a 20-bit memory address, allowing it to access the start of a segment. The microprocessor must generate a 20-hit memory address to access a location within the first 1 M of memory. For example, when a segment register contains a 1200H, it addresses a 64K-byte memory segment beginning at location 12000H. Likewise, if a segment register contains a 1201H, it addresses a memory segment beginning at location 12010H. Because of the internally ap­pended 0H, real mode segments can begin only at a 16-byte boundary in the memory system. This 16-byte boundary is often called a paragraph.

















Because a real mode segment of memory is 64K in length, once the beginning address is known, the ending address is found by adding FFFFH.

The offset address, which is a part of the address, is added to the start of the segment to ad­dress a memory location within the memory segment. For example, if the segment address is 1000H and the offset address is 2000H, the microprocessor addresses memory location 12000H. The offset address is always added to the starting address of the segment to locate the data. The segment and offset address is sometimes written as 1000:2000 for a segment address of 1000H with an offset of 2000H.

In the 80286 (with special external circuitry), and the 80386 through the Pentium II, an extra 64K minus 16 bytes of memory is addressable when the segment address is FFFFH and the HIMEM.SYS driver is installed in the system. This area of memory (0FFFF0H-10FFEFH) is re­ferred to as high memory.

Some addressing modes combine more than one register and an offset value to form an offset address. When this occurs, the sum of these values may exceed FFFFH. For example, the address accessed in a segment whose segment address is 4000H, and whose offset address is specified as the sum of F000H plus 3000H, will access memory location 42000H instead of lo­cation 52000H. When the F000H and 3000H are added, they form a 16-bit (modulo 16) sum of 2000H used as the offset address; not 12000H, the true sum. Note that the carry of 1 (F000H + 3000H=12000H) is dropped for this addition to form the offset address of 2000H. This means that the address is generated as 4000:2000 or 42000H.


2.2.2.  Default Segment and Offset Registers


         The microprocessor has a set of rules that apply to segments whenever memory is addressed, These rules, which apply in the real and protected mode, define the segment register and offset register combination. For example, the code segment register is always used with the instruction pointer to address the next instruction in a program. This combination is CS:IP or CS:EIP, de­pending upon the microprocessor’s mode of operation. The code segment register defines the start of the code segment and the instruction pointer locates the next instruction within the code segment. This combination (CS:IP or CS:EIP) locates the next instruction executed by the mi­croprocessor.

Another of the default combinations is the stack. Stack data are referenced through the stack segment at the memory location addressed by either the stack pointer (SP/ESP) or the base pointer (BP/EBP). These combinations are referred to as SS:SP (SS:ESP) or SS:BP (SS:EBP). Note that in real mode, only the rightmost 16 bits of the extended register address a location within the memory segment. In the 80386—Pentium II, never place a number larger than FFFFH into an offset register if the microprocessor is operated in the real mode. This causes the system to halt and indicate an addressing error.

Other defaults are shown in Table 2-1 for addressing memory using any Intel micropro­cessor with 16-bit registers. Table 2-2 shows the defaults assumed in the 80386 and above when using 32-bit registers. Note that the 80386 and above have a far greater selection of segment offset address combinations than do the 8086 through the 80286 microprocessors.



















The 8086-80286 microprocessors allow four memory segments and the 80386 and above allow six memory segments. Figure 2-4 shows a system that contains four memory segments. Note that a memory segment can touch or even overlap if 64K bytes of memory are not required for a segment. Think of segments as windows that can be moved over any area of memory to ac­cess data or code. Also note that a program can have more than four or six segments, but can only access four or six segments at a time.

Suppose that an application program requires 1000H bytes of memory for its code, 190H bytes of memory for its data, and 200H bytes of memory for its stack. This application does not require an extra segment. When this program is placed in the memory system by DOS, it is loaded in the TPA at the first available area of memory above the drivers and other TPA pro­grams. This area is indicated by a free-pointer that is maintained by DOS. Program loading is handled automatically by the program loader located within DOS. Figure 2-5 shows how this application is stored in the memory system. The segments show an overlap because the amount of data in them does not require 64K bytes of memory. The side view of the segments clearly shows the overlap. It also shows how segments can be moved over any area of memory by changing the segment starting address. Fortunately, the DOS program loader calculates and as­signs segment starting addresses.











2.2.3.  Segment and Offset Addressing Scheme Allows Relocation


         The segment and offset addressing scheme seems unduly complicated. It is complicated, but it also affords an advantage to the system. This complicated scheme of segment plus offset ad­dressing allows programs to be relocated in the memory system. It also allows programs written to function in the real mode to operate in a protected mode system. A relocatable pro­gram is one that can be placed into any area of memory and executed without change. Relocat­able data are data that can be placed in any area of memory and used without any change to the program. The segment and offset addressing scheme allows both programs and data to be relo­cated without changing a thing in a program or data. This is ideal for use in a general-purpose computer system in which not all machines contain the same memory areas. The personal computer memory structure is different from machine to machine, requiring relocatable soft­ware and data.

Because memory is addressed within a segment by an offset address, the memory segment can be moved to any place in the memory system without changing any of the offset addresses. This is accomplished by moving the entire program, as a block, to a new area and then changing only the contents of the segment registers. If an instruction is 4 bytes above the start of the seg­ment, its offset address is 4. If the entire program is moved to a new area of memory, this offset address of 4 still points to 4 bytes above the start of the segment. Only the contents of the seg­ment register must be changed to address the program in the new area of memory. Without this feature, a program would have to be extensively rewritten or altered before it is moved. This would require additional time or many versions of a program for the many different configura­tions of computer systems.


2.3.        Introduction to Protected Mode Memory Addressing


         Protected mode memory addressing (80286 and above) allows access to data and programs lo­cated above the first 1M byte of memory, as well as within the first 1M byte of memory. Ad­dressing this extended section of the memory system requires a change to the segment plus an offset addressing scheme used with real mode memory addressing. When data and programs are addressed in extended memory, the offset address is still used to access information located within the memory segment. One difference is that the segment address, is no longer present in the protected mode. In place of the segment ad­dress, the segment register contains a selector that selects a descriptor from a descriptor table. The descriptor describes the memory segment’s location, length, and access rights. Because the segment register and offset address still access memory, protected mode instructions are iden­tical to real mode instructions. In fact, most programs written to function in the real mode will function without change in the protected mode. The difference between modes is in the way that the segment register is interpreted by the microprocessor to access the memory segment. An­other difference, in the 80386 and above, is that the offset address can be a 32-bit number instead of a 16-bit number in the protected mode. A 32-bit offset address allows the microprocessor to access data within a segment that can be up to 4G bytes in length.


2.3.1.     Selectors And Descriptors


         The selector, located in the segment register, selects one of 8192 descriptors from one of two ta­bles of descriptors. The descriptor describes the location, length, and access rights of the seg­ment of memory. Indirectly, the segment register still selects a memory segment, but not directly as in the real mode. For example, in the real mode, if CS = 0008H, the code segment begins at lo­cation 00080H. In the protected mode, this segment number can address any memory location in the entire system for the code segment.

   There are two descriptor tables used with the segment registers: one contains global de­scriptors and the other contains local descriptors. The global descriptors contain segment defini­tions that apply to all programs, while the local descriptors are usually unique to an application. You might call a global descriptor a system descriptor and call a local descriptor an application descriptor. Each descriptor table contains 8192 descriptors, so a total of 16,384 total descriptors are available to an application at any time. Because the descriptor describes a memory segment, this allows up to 16,384 memory segments to be described for each application.

Figure 2-6 shows the format of a descriptor for the 80286 through the Pentium II. Note that each descriptor is 8 bytes in length, so the global and local descriptor tables are each a max­imum of 64K bytes in length. Descriptors for the 80286 and the 80386 through the Pentium II differ slightly, but the 80286 descriptor is upward-compatible.




The base address portion of the descriptor indicates the starting location of the memory segment. For the 80286 microprocessor, the base address is a 24-bit address, so segments begin at any location in its 16M bytes of memory. Note that the paragraph boundary limitation is re­moved in these microprocessors when operated in the protected mode. The 80386 and above use a 32-bit base address that allows segments to begin at any location in its 4G bytes of memory. Notice how the 80286 descriptor’s base address is upward-compatible to the 80386 through the Pentium II descriptor because its most-significant 16 bits are 0000H.  

The segment limit contains the last offset address found in a segment. For example, if a segment begins at memory location F00000H and ends at location F000FFH, the base address is F00000H and the limit is FFH. For the 80286 microprocessor, the base address is F00000H and the limit is OOFFH. For the 80386 and above, the base address is 00F00000H and the limit is 000FFH. Notice the limit the 80286 has a 16-bit limit and the 80386 through the Pentium II have a 20-bit limit. The 80286 accesses memory segments that are between 1 and 64K bytes in length. The 80386 and above access memory segments that are between 1 and 1 M byte, or 4K and 4G bytes in length.

There is another feature found in the 80386 through the Pentium II descriptor that is not found in the 80286 descriptor: the G bit, or granularity bit. If G=0, the limit specifies a seg­ment limit of 00000H to FFFFFH. If G = 1, the value of the limit is multiplied by 4K bytes (ap­pended with XXXH). The limit is then 00000XXXH to FFFFFXXXH, if G=1. This allows a segment length of 4K to 4G bytes in steps of 4K bytes. The reason that the segment length is 64K bytes in the 80286 is that the offset address is always 16 bits because of its 16-bit internal archi­tecture. The 80386 and above use a 32-bit architecture that allows an offset address, in the pro­tected mode operation, of the 32 bits. This 32-bit offset address allows segment lengths of 4G bytes and the 16-bit offset address allows segment lengths of 64K bytes. Operating systems op­erate in a 16 or 32-bit environment. For example, DOS uses a 16-bit environment, while most Windows applications use a 32-bit environment.

The AV bit, in the 80386 and above descriptor, is used by some operating systems to indi­cate that the segment is available (AV = 1) or not available (AV = 0). The D bit indicates how the 80386 through the Pentium II instructions access register and memory data in the protected or real mode. If D=0, the instructions are 16-bit instructions, compatible with the 8086—80286 mi­croprocessors. This means that the instructions use 16-bit offset addresses and 16-bit registers by default. This mode is often called the 16-bit instruction mode. If D=1, the instructions are 32-bit instructions. By default, the 32-bit instruction mode assumes that all offset addresses and all registers are 32 bits. Note that the default for register size and offset address size can be overridden in both the 16- and 32-bit instruction modes. Both the MSDOS and PCDOS oper­ating systems require that the instructions are always used in the 16-bit instruction mode. Win­dows 3.1 also requires that the 16-bit instruction mode is selected. Note that the 32-bit instruction mode is accessible only in a protected-mode system such as Windows NT, Windows 95, Windows 98, or OS/2.

 The access rights byte in Figure 2-7 controls access to the protected mode memory segment. This byte describes how the segment functions in the system. The access rights byte al­lows complete control over the segment. If the segment is a data segment, the direction of growth is specified. If the segment grows beyond its limit, the microprocessor’s program is interrupted, indicating a general protection fault. You can even specify whether a data segment can be written or is write-protected. The code segment is also controlled in a similar fashion and can have reading inhibited to protect software.

Descriptors are chosen from the descriptor table by the segment register. Figure 2-8 shows how the segment register functions in the protected mode system. The segment register contains a 13-bit selector field, a table selector bit, and a requested privilege level field. The 13-bit selector chooses one of the 8192 descriptors from the descriptor table. The TI bit selects ei­ther the global descriptor table (TI = 0) or the local descriptor table (TI = 1). The requested privilege level (RPL) requests the access privilege level of a memory segment. The highest privilege level is 00 and the lowest is 11. If the requested privilege level matches or is higher in priority than the privilege level set by the access rights byte, access is granted. For example, if the requested privilege level is 10 and the access rights byte sets the segment privilege level at 11, access is cvranted because 10 is higher in priority than privilege level 11. Privilege levels are used in multiuser environments. If the privilege level is violated, the system normally indicates a privilege violation.


Figure 2-9 shows how the segment register, containing a selector, chooses a descriptor from the global descriptor table. The entry in the global descriptor table selects a segment in the memory system. In this illustration, DS contains 0008H, which accesses the descriptor number 1 from the global descriptor table by using a requested privilege level of 00. Descriptor number 1 contains a descriptor that defines the base address as 00100000H with a segment limit of 000FFH. This means that a value of 0008H loaded into DS causes the microprocessor to use memory locations 00100000H—001000FFH for the data segment with this example descriptor table. Note that de­scriptor zero is called the null descriptor and may not be used for accessing memory.





2.3.2.  Program-Invisible Registers


         The global and local descriptor tables are found in the memory system. In order to access and specify the address of these tables, the 80286, 80386, 80486, Pentium, Pentium Pro, and Pentium II contain program-invisible registers. The program-invisible registers are not directly addressed by software so they are given this name (although some of these registers are accessed by the system software). Figure 2-10 illustrates the program-invisible registers as they appear in the 80286 through the Pentium II. These registers control the microprocessor when operated in the protected mode.

   Each of the segment registers contains a program-invisible portion used in the protected mode. The program-invisible portion of these registers is often called cache memory because a cache is any memory that stores information. This cache is not to be confused with the normal level 1 or level 2 caches found with the microprocessor. The program-invisible portion of the seg­ment register is loaded with the base address, limit, and access rights each time the number in the segment register is changed. When a new segment number is placed in a segment register, the mi­croprocessor accesses a descriptor table and loads the descriptor into the program-invisible cache portion of the segment register. It is held there and used to access the memory segment until the segment number is again changed. This allows the microprocessor to repeatedly access a memory segment without referring to the descriptor table for each access (hence the term cache).

The GDTR (global descriptor table register) and IDTR (interrupt descriptor table reg­ister) contain the base address of the descriptor table and its limit. The limit of each descriptor table is 16 bits because the maximum table length is 64K bytes. When the protected mode oper­ation is desired, the address of the global descriptor table and its limit are loaded into the GDTR.

Before using the protected mode, the interrupt descriptor table and the IDTR must also be ini­tialized. More detail is provided on protected mode operation later in the text. At this point, the programming and additional description of these registers are impossible.

The location of the local descriptor table is selected from the global descriptor table. One of the global descriptors is set up to address the local descriptor table. To access the local descriptor table, the LDTR (local descriptor table register) is loaded with a selector, just as a segment reg­ister is loaded with a selector. This selector accesses the global descriptor table and loads the base address, limit, and access rights of the local descriptor table into the cache portion of the LDTR.

The TR (task register) holds a selector, which accesses a descriptor that defines a task. A task is most often a procedure or application program. The descriptor for the procedure or applica­tion program is stored in the global descriptor table, so access can be controlled through the privi­lege levels. The task register allows a context or task switch in about 17 µs. Task switching allows the microprocessor to switch between tasks in a fairly short amount of time. The task switch allows multitasking systems to switch from one task to another in a simple and orderly fashion.







2.4.     Memory Paging


            The memory paging mechanism located within the 80386 and above allows any physical memory location to be assigned to any linear address. The linear address is defined as the address generated by a program. With the memory paging unit, the linear address is invisibly translated into any physical address, which allows an application written to function at a specific address to be re­located through the paging mechanism. It also allows memory to be placed into areas where no memory exists. An example is the upper memory blocks provided by EMM386.EXE.

The EMM386.EXE program reassigns extended memory, in 4K blocks, to the system memory between the video BIOS and the system BIOS ROMS for upper memory blocks. With­out the paging mechanism, the use of this area of memory is impossible.


2.4.1.  Paging Registers


         The paging unit is controlled by the contents of the microprocessor’s control registers. See Fig­ure 2-11 for the contents of control registers CR0 through CR3. Note that these registers are only available to the 80386 through the Pentium microprocessors. Beginning with the Pentium, an addi­tional control register labeled CR4 controls extensions to the basic architecture provided in the Pen­tium and above microprocessors. One of these features is a 4M-byte page that is enabled by setting bit position 4, or CR4.

   The registers important to the paging unit are CR0 and CR3. The leftmost bit (PG) position of CR0 selects paging when placed at a logic 1 level. If the PG bit is cleared (0), the linear ad­dress generated by the program becomes the physical address used to access memory. If the PG bit is set (1), the linear address is converted to a physical address through the paging mechanism. The paging mechanism functions in both the real and protected modes.

   CR3 contains the page directory base address, and the PCD and PWT bits. The PCD and PWT bits control the operation of the PCD and PWT pins on the microprocessor. If PCD is set (1), the PCD pin becomes a logic one during bus cycles that are not pages. This allows the external hardware to control the level 2 cache memory. (Note that the level 2 cache memory is an external high-speed memory that functions as a buffer between the microprocessor and the main DRAM memory system.) The PWT bit also appears on the PWT pin, during bus cycles that are not pages, to control the write-through cache in the system. The page directory base address locates the page directory for the page translation unit. Note that this address locates the page directory at any 4K boundary in the memory system because it is appended internally with a 000H. The page directory contains 1024 directory entries of 4 bytes each. Each page directory entry addresses a page table that contains 1024 entries.


The linear address, as it is generated by the software, is broken into three sections that are used to access the page directory entry, page table entry, and page offset address. Figure 2-12 shows the linear address and its makeup for paging. Notice how the leftmost 10 bits address an entry in the page directory. For linear address 00000000H—003FFFFFH, the first entry of the page directory is accessed. Each page directory entry represents or repages a 4M-byte section of the memory system. The contents of the page directory select a page table that is indexed by the next 10 bits of the linear address (bit positions 12-21). This means that address 00000000H— 00000FFFH selects page directory entry 0 and page table entry 0. Notice this is a 4K-byte ad­dress range. The offset part of the linear address (bit positions 0-11) next selects a byte in the 4K-byte memory page. In Figure 2-12, if the page table 0 entry contains address 00100000H, then the physical address is 00100000H-00100FFFH for linear address 00000000H-00000FFFH. This means that when the program accesses a location between 00000000H and 00000FFFH, the microprocessor physically addresses location 00100000H—00100FFFH.

Because the act of repaging a 4K-byte section of memory requires access to the page di­rectory and a page table, which are both located in memory, Intel has incorporated a cache called the TLB (translation look-aside buffer). In the 80486 microprocessor, the cache holds the 32 most recent page translation addresses. This means that the last 32 page table translations are stored in the TLB, so if the same area of memory is accessed, the address is already present in the TLB, and access to the page directory and page tables is not required. This speeds program exe­cution. If a translation is not in the TLB, the page directory and page table must be accessed, which requires additional execution time. The Pentium, Pentium Pro, and Pentium II contain separate TLBs for each of their instruction and data caches.




2.4.2.  The Page Directory and Page Table


         Figure 2-13 shows the page directory, a few page tables, and some memory pages. There is only one page directory in the system. The page directory contains 1024 doubleword addresses that locate up to 1024 page tables. The page directory and each page table are 4K bytes in length. If the entire 4G byte of memory is paged, the system must allocate 4K bytes of memory for the page directory, and 4K times 1024 or 4M bytes for the 1024 page tables. This represents a con­siderable investment in memory resources.


The DOS system and EMM386.EXE use page tables to redefine the area of memory be­tween locations C8000H—EFFFFH as upper memory blocks. It does this by repaging extended memory to back-fill this part of the conventional memory system to allow DOS access to addi­tional memory. Suppose that the EMM386.EXE program allows access to 16M bytes of ex­tended and conventional memory through paging and locations C8000H—EFFFFH must be repaged to locations 110000—138000H, with all other areas of memory paged to their normal lo­cations. Such a scheme is depicted in Figure 2-14.

Here, the page directory contains four entries. Recall that each entry in the page directory corresponds to 4M bytes of physical memory. The system also contains four page tables with 1024 entries each. Recall that each entry in the page table repages 4K bytes of physical memory. This scheme requires a total of 16K of memory for the four page tables and 16 bytes of memory for the page directory.

As with DOS, the Windows program also repages the memory system. At present, Windows version 3.11 supports paging for only l6M bytes of memory because of the amount of memory re­quired to store the page tables. On the Pentium and Pentium Pro microprocessors, pages can be ei­ther 4K bytes in length or 4M bytes in length. Although no software currently supports the 4M-byte pages, as the Pentium II and more advanced versions pervade the personal computer, operating sys­tems of the future will undoubtedly begin to support 4M-byte memory pages.

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                        Figure 2-14 : The page directory, page table 0, and two memory pages.









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                        Figure 2-15 : The page directory, and page table.