Chapter2.3 4-mikroprocessor

The Microprocessor and Its The Microprocessor and Its ArchitectureArchitecture

A Course in MicroprocessorElectrical Engineering Department

University of Indonesia

Protected Mode Protected Mode Memory AddressingMemory Addressing

• Protected mode memory addressing (80286 and

above) allows access to data and programs located above the first 1MB of memory as well as within the first 1MB of memory

• The segment register contains a selector instead of segment addresses– a selector selects a descriptor from descriptor

table– The descriptor describes the memory segment’s

location, length, and access rights (Fig.2.6)

Protected Mode Memory Protected Mode Memory Addressing (cont’d)Addressing (cont’d)

– The selector selects one of 8,192 descriptors from one of two tables of descriptors

– There are two descriptor tables: one contains global descriptors and other contains local descriptors

– The global descriptors contain segment definitions that apply to all programs, while the local des-criptors are usually unique to an applications

– Each descriptor table contains 8,192 descriptors (i.e. a total 16,384 descriptors are available to an application at any time) --> eq. 16,834 memory segment

Protected Mode Memory Protected Mode Memory Addressing (cont’d)Addressing (cont’d)

– The base address portion of the descriptor indicates the starting location of the memory segment

– The segment limit contains the last offset address found in a segment

– The G bit (Granularity; for 80386 through Pen-tium Pro) specifies a segment limit of from 1B to 1MB in length if G = 0, and any multiple of 4KB if G = 1

– The AV bit is used by some operating systems to indicate that the segment is available (AV=1) or not available (AV=0)

Protected Mode Memory Protected Mode Memory Addressing (cont’d)Addressing (cont’d)

– The D bits indicates whether the instructions to access register and memory data is 16-bit instructions (D=0) or 32-bit instructions (D=1)

– The access rights bytes (Fig.2.7) controls access to the protected mode memory segment

• Descriptors are chosen from table by the segment register– Fig. 2.8 shows how the segment register functions in the

protected mode system– Fig.2.9 shows how the segment register, containing a

selector, chooses a descriptor from the global descriptor table

Program Invisible RegistersProgram Invisible Registers

• The program invisible are not directly addressed by software (i.e. invisible)

• Figures 2.10 Illustrates the program-invisible registers– These registers control the microprocessor when

operated in the protected mode– The program-invisible portion of these registers

is often called cache memory; do not confuse with the normal level 1 or level 2 caches found in the microprocessor

Program Invisible RegistersProgram Invisible Registers

• The GDTR (global descriptor table register) and IDTR (interrupt descriptor table register) contain the base address of the descriptor table and its limit (i.e. 16-bits)– the location of the local descriptor table is

selected from the global descriptor table– One of the global descriptor is set up to address

the local descriptor table

Memory PagingMemory Paging

• The memory paging mechanism located within the 80386 and above allows any physical memory location to be assign to any linear address– The linear address is defined as the address

generated by a program, and with the memory paging unit, the linear address is invisibly translated into any physical address

• This allows an application written to function at a specific address to be relocated through the paging mechanism

Memory Paging (cont’d)Memory Paging (cont’d)

• It also allows memory to be placed into areas where no memory exists

• The paging unit is controlled by the contents of the microprocessor’s control registers (CR0 - CR3 or CR4, Figure 2.11)

• The linear address is broken into three sections that are used to access the page directory entry, page table entry, and page offset address (Figure.2.12)

Memory Paging (cont’d)Memory Paging (cont’d)

• See Figure.2.13– The page directory contains 1,024 doubleword

addresses that locate up to 1,024 page tables– The page directory and each page table are 4K

bytes in length– See Figure.2.14 as well