32 bit MicroprocessorIntel 80386 30 Marks Course Outcome

32 - bit Microprocessor-Intel 80386 30 Marks Course Outcome: Explain memory management and concept of pipelining. Describe the concept of paging and addressing. Visit for more Learning Resources

Signal Description of 80386

Signal Descriptions of 80386 • W/R#: The write / read output distinguishes the write and read cycles from one another. • D/C#: This data / control output pin distinguishes between a data transfer cycle from a machine control cycle like interrupt acknowledge. • M/IO#: This output pin differentiates between the memory and I/O cycles. • LOCK#: The LOCK# output pin enables the CPU to prevent the other bus masters from gaining the control of the system bus.

• NA#: The next address input pin, if activated, allows address pipelining, during 80386 bus cycles. • ADS#: The address status output pin indicates that the address bus and bus cycle definition pins( W/R#, D/C#, M/IO#, BE 0# to BE 3# ) are carrying the respective valid signals. The 80383 does not have any ALE signals and so this signals may be used for latching the address to external latches. • READY#: The ready signals indicates to the CPU that the previous bus cycle has been terminated and the bus is ready for the next cycle. The signal is used to insert WAIT states in a bus cycle and is useful for interfacing of slow devices with CPU. • VCC: These are system power supply lines. • VSS: These return lines for the power supply

• BS 16#: The bus size – 16 input pin allows the interfacing of 16 bit devices with the 32 bit wide 80386 data bus. Successive 16 bit bus cycles may be executed to read a 32 bit data from a peripheral. • HOLD: The bus hold input pin enables the other bus masters to gain control of the system bus if it is asserted. • HLDA: The bus hold acknowledge output indicates that a valid bus hold request has been received and the bus has been relinquished by the CPU. • BUSY#: The busy input signal indicates to the CPU that the coprocessor is busy with the allocated task.

• ERROR#: The error input pin indicates to the CPU that the coprocessor has encountered an error while executing its instruction. • PEREQ: The processor extension request output signal indicates to the CPU to fetch a data word for the coprocessor. • INTR: This interrupt pin is a maskable interrupt, that can be masked using the IF of the flag register. • NMI: A valid request signal at the non-maskable interrupt request input pin internally generates a non-maskable interrupt of type 2.

• RESET: A high at this input pin suspends the current operation and restart the execution from the starting location. • N / C: No connection pins are expected to be left open while connecting the 80386 in the circuit.

Architecture of 80386

Architecture of 80386 • The Internal Architecture of 80386 is divided into 3 sections. – Central Processing Unit • Execution Unit • Instruction Unit – Memory management unit • Segmentation • Paging Unit – Bus interface unit

• The execution unit consists of the eight 32 bit general purpose registers (GPR) which are use in physical address generation, to hold 8 or 16 or 32 bit data and a 64 - bit barrel shifter used to increase the speed of shift, rotate, multiply and divide operations. • The instruction unit decodes the op-code of the instructions which is read from the memory and stores them in the decoded instruction que for immediate use by the execution unit during execution.

• The memory Management Unit (MMU) has two sub unit i. e. Segmentation Unit and Paging Unit. • Memory is divided into one or more variable length segments and each 64 Kbytes or 4 Gigabytes in size depending on memory management techniques used either segmentation or paging. • Each task on 80386 can have a maximum of 16, 381 segments of up to 4 GB each, thus providing 64 TB of virtual memory to each task. • In segmentation unit 80386 provides four level protection for protecting user program and the operating system from each other.

• The 80386 has three modes of operation: 1. Real Address Mode (Real Mode) 2. Protected Virtual Addressing mode (Protected Mode) 3. Virtual 8086 mode. • The BIU also offers address pipelining, data bus sizing, and direct Byte Enable signals for each byte of data bus.

Register of 80386 • The 80386 has 32 registers in the following categories: – – – – General Purpose Register Segment Registers Instruction Pointer and Flags Status and Control Registers System Address Registers Debug Registers Test Registers

Memory Organization in 80386

• Address Space in 80386 • IO Space in 80386

Real Addressing Modes of 80386 • When processor Reset or Powered up then 80386 is initialized in REAL Mode. • Real mode has same base architecture as that of 8086, but allows access to the 32 -bit register set of 80386. • The addressing mechanism, memory size, interrupt handelling , are identical to Real Mode of the 80286. • All of the 80386 instructions are available in Real Mode is 16 -bits, same as that of 8086. • To use 32 -bit registers and addressing mmodes, override prefixes ‘E’ must be used. • The segment size on the 80386 in Real mode is 64 KB so 32 -bit effective addresses must be less than 0000 FFFFH. • The basic Purpose of Real mode in 80386 is to set up the processor for Protected Mode Operation.

Memory Addressing in Real Mode of 80386 • In Real mode the max. memory size is limited to 1 MB, so only address line A 2 -A 19 are used. • In Real addressing mode the linear addresses are the same physical addresses as paging is not allowed. • Physical address is calculated in Real mode by adding the content of segment register after shifting it left by four bits with an effective address and generates a physical address. Which is compatible with 80286 Real Mode.

• All address in Real Mode should not more than 64 KB in size and may be Read, written or executed. • The 80386 generates an exception 13 if a data operand or instruction fetch occurs past the end of a segment. • Segment may be overlapped in Real mode means if a particular segment does not use all 64 KB another segment can be override on top of the unused portion of the previous segment. • Hence, this allows the programmer to minimize the amount of physical memory needed for a program.

Protected Virtual Address Mode (PVAM) of 80386 • When the processor operates in protected virtual address mode i. e. Protected Mode then the complete capabilities of the 80386 are used. • Protected mode is used to increase the linear address space up to 4 GB and used to execute virtual memory programs of the size of 64 TB. • Also Protected mode of 80386 is used to run all the existing 8086 and 80286 program with a memory management and protection mechanism. • Protected mode of 80386 also supports the use of additional instructions for supporting multitasking OS.

• The linear address is then either used as the 32 - bit physical address, or the paging mechanism maps the 32 - bit linear address into a 32 bit physical address if paging is enabled. • In protected mode the selector is used as an index for system define table such as LDT or GDT • The LDT or GDT table contains 32 bit base address of given segment. • The physical address is formed by adding the base address obtained from the table to offset. • Paging provides additional memory management mechanism which operates only in Protected mode and managing the very large segments of the 80386.

Segment Descriptor Cache • Every segment register has a segment descriptor cache register. • Selector fields: In protected mode of 80386, a selector has three fields i. e. Table indicator (TI) which is used to points either a Local or Global descriptor table, descriptor entry index (Index) and Requestor Privilege Level (RPL) as shown in fig. • The TI bit is used to select one of two tables of descriptors. i. e. GDT and if TI bit is 0 • And TI is 1 then LDT

• The Index is used to selects one of 8 KB descriptors entry in appropriate descriptor table. • The RPL (Requestor Privilege Level) bits are used for testing of the selector’s privilege attributes.

Descriptor Attributes Bits

Detailed Format of Segments Descriptor cache Register • Segment Descriptor Cache in Real Mode: Internal Privilege level is always fixed to highest level, level 0, so I/O and other privilrged op-codes can be executed. • Segment Descriptor Cache in Protected Mode: All fields are defined by the content of segment descriptor pointed by the selector index value in the segment register. • Segment Descriptor Cache in Virtual Mode: The virtual program executes at lowest privilege level, i. e. level 3, to allow trapping of all IOPL sensitive instructions and level-0 only instruction.

Difference between Real and Protected Mode Real Mode Protected Mode (PVAM) Memory addressing up to 1 MB physical memory Memory addressing up to 16 MB of physical memory No virtual memory support Supports up tp to 64 TB of virtual memory Memory Protection mechanism is not available Memory Protection Mechanism is avilable Does not support virtual address space Gives virtual and physical address space Does not support LDT and GDT Supports LDT and GDT Segment descriptor cache is not available Segment descriptor cache is available Supports Segmentation Supports segmentation and paging.

Segmentation • Segmentation is memory management technique with protection. • The information of the segment is stored in an 8 byte format called as descriptor. • There are three types of descriptor table: – LDT – GDT – IDT • LDT, GDT and IDT are of variable length memory size table which ranges between 8 bytes and 64 KB.

• Each table content maximum 8 K od 8 byte descriptors and the upper 13 bits of selectors are used as an index for the descriptor table. • These tables have registers associated with them which hold the 32 -bit linear base address and the 16 - bit limit of each table and these registers are LDTR, GDTR and IDTR.

Protection Levels in 80386 • The 80386 supports four protection which isolates and protect user programs for each other and the operating system during a multi-tasking operating system. • The protection level avoid the use of privileged instructions, I/O instructions and access to segments and segment descriptors. • The privilege protection mechanism of 80386 is integrated in On- chip Memory Management Unit which also gives protection to pages, when paging is enable.

Privilege levels in 80386 A. Task Privilege level: • • At any point in time, a task in 80386 always executes at one of four Privilege levels. The current privilege level of selector is indicated by the RPL field. i. e. the two least significant bits of the selector. B. Selector Privilege (RPL): • • The privilege level of selector is indicated by the RPL field i. e. two least significant bits of the selector. RPL of selector is used only to test requestor privilege level with current privilege level of a segment to be used called effective privilege level i. e. EPL

Virtual 8086 mode for executing 80386 Programs

• The 80386 allows the execution of 8086 program in both Real Mode and Virtual 8086 mode i. e. Virtual Mode. • Virtual 8086 mode provides the system designer the most flexibility out of two modes i. e Real and PVAM. • The virtual mode allows the execution 8086 programs, by taking full advantage of the 80386 protection mechanism. • Hence, it allows the simultaneous exection of 8086 OS with its application, and 80386 OS, both 80286 and 80386 applications.

Virtual 8086 Mode Addressing Mechanism • The major difference between 80386 Real and Protected mode is the way that segment selectors are interpreted. • When the processor is operating in Virtual Mode the segment registers are used in an identical to Real Mode. • The content of the segment register are shifted towards left by 4 bits and added to the offset to form the segment base linear address. • The 80386 gives facility to the OS for specifying which programs use 8086 style address mechanism, and which program use Protected mode addressing on a per task basis.

• By the use of paging memory management, the one MB address space of virtual mode task can be mapped anywhere in the 4 GB of linear address space of 80386. • Virtual mode effective address i. e. segment offsets that exceed 64 KB will cause an exception 13 as in Real mode. For more detail contact us