INTRODUCTION TO THE 80286
The 80286 microprocessor is an advanced version of the 8086 microprocessor that was designed for multiuser and multitasking environments. The 80286 addresses 16M bytes of physical memory and 1G bytes of virtual memory by using its memory-management system. This section of the text introduces the 80286 microprocessor, which finds use in earlier AT-style personal computers that once pervaded the computer market and still find some applications. The 80286 is basically an 8086 that is optimized to execute instructions in fewer clocking periods than the 8086. The 80286 is also an enhanced version of the 8086 because it contains a memory manager. At this time, the 80286 no longer has a place in the personal computer system, but it does find applications in control systems as an embedded controller.
Hardware Features
Figure 16–29 shows the internal block diagram of the 80286 microprocessor. Note that like the 80186/80188, the 80286 does not incorporate internal peripherals; instead, it contains a memory- management unit (MMU) that is called the address unit in the block diagram.
As mentioned in Chapter 1, the 80286 operates in both the real and protected modes. In the real mode, the 80286 addresses a 1M-byte memory address space and is virtually identical to the 8086. In the protected mode, the 80286 addresses a 16M-byte memory space.
Figure 16–31 illustrates the basic 80286 microprocessor-based system. Notice that the clock is provided by the 82284 clock generator (similar to the 8284A) and the system control signals are provided by the 82288 system bus controller (similar to the 8288). Also, note the absence of the latch circuits used to demultiplex the 8086 address/data bus.
Additional Instructions
The 80286 has even more instructions than its predecessors. These extra instructions control the virtual memory system through the memory manager of the 80286. Table 16–9 lists the additional 80286 instructions with a comment about the purpose of each instruction. These instructions are the only new instructions added to the 80286. Note that the 80286 contains the new instructions added to the 80186/80188 such as INS, OUTS, BOUND, ENTER, LEAVE, PUSHA, POPA, and the immediate multiplication and immediate shift and rotate counts.
Following are descriptions of instructions not explained in the memory-management section. The instructions described here are special and only used for the conditions indicated.
CLTS The clear task-switched flag (CLTS) instruction clears the TS (task-switched) flag bit to a logic 0. If the TS flag bit is a logic 1 and the 80287 numeric coprocessor is used by the task, an interrupt occurs (vector type 7). This allows the function of the coprocessor to be emulated with software. The CLTS instruction is used in a system and is considered a privileged instruction because it can be executed only in the protected mode at privilege level 0. There is no set TS flag instruction; this is accomplished by writing a logic 1 to bit position 3 (TS) of the machine status word (MSW) by using the LMSW instruction.
LAR The load access rights (LAR) instruction reads the segment descriptor and places a copy of the access rights byte into a 16-bit register. An example is the LAR AX,BX instruction that loads AX with the access rights byte from the descriptor selected by the selector value found in BX. This instruction is used to get the access rights so that it can be checked before a program uses the segment of memory described by the descriptor.
LSL The load segment limit (LSL) instruction loads a user-specified register with the segment limit. For example, the LSL AX,BX instruction loads AX with the limit of the segment described by the descriptor selected by the selector in BX. This instruction is used to test the limit of a segment.
ARPL The adjust requested privilege level (ARPL) instruction is used to test a selector so that the privilege level of the requested selector is not violated. An example is ARPL AX,CX: AX contains the requested privilege level and CX contains the selector value to be used to access a descriptor. If the requested privilege level is of a lower priority than the descriptor under test, the zero flag is set. This may require that a program adjust the requested privilege level or indicate a privilege violation.
VERR The verify for read access (VERR) instruction verifies that a segment can be read.
Recall from Chapter 1 that a code segment can be read-protected. If the code segment can be read, the zero flag bit is set. The VERR AX instruction tests the descriptor selected by the AX register.
VERW The verify for write access (VERW) instruction is used to verify that a segment can be written. Recall from Chapter 1 that a data segment can be write-protected. If the data segment can be written, the zero flag bit is set.
The Virtual Memory Machine
A virtual memory machine is a machine that maps a larger memory space (1G bytes for the 80286) into a much smaller physical memory space (l6M bytes for the 80286), which allows a very large system to execute in smaller physical memory systems. This is accomplished by spooling the data and programs between the fixed disk memory system and the physical memory. Addressing a 1G-byte memory system is accomplished by the descriptors in the 80286 micro- processor. Each 80286 descriptor describes a 64K-byte memory segment and the 80286 allows 16K descriptors. This (64K × 16K) allows a maximum of 1G bytes of memory to be described for the system.
As mentioned in Chapter 1, descriptors describe the memory segment in the protected mode. The 80286 has descriptors that define codes, data, stack segments, interrupts, procedures, and tasks. Descriptor accesses are performed by loading a segment register with a selector in the protected mode. The selector accesses a descriptor that describes an area of the memory. Additional details on descriptors and their applications are defined in Chapter 1, and also Chapters 17, 18, and 19. Please refer to these chapters for a detailed view of the protected mode memory-management system.