80486 System Architecture [3rd Edition] [1995]

Contents About This Book The MindShare Architecture Series.......................................................................................1 Organization of This Book.......................................................................................................2 Who Should Read This Book ..................................................................................................3 Prerequisite Knowledge...........................................................................................................4 Documentation Conventions...................................................................................................4 Hex Notation.......................................................................................................................4 Binary Notation....................................................................................................................4 Decimal Notation.................................................................................................................4 Signal Name Representation..............................................................................................5 Identification of Bit Fields...................................................................................................5 We Want Your Feedback ..........................................................................................................5 E-Mail/Phone/FAX ............................................................................................................5 Bulletin Board.......................................................................................................................6 Mailing Address ..................................................................................................................6 Chapter 1: 80486 Overview System Performance Prior to the 80486..................................................................................7 The Memory Bottleneck ...........................................................................................................7 The Static Ram, or SRAM, Solution...................................................................................8 The External Cache Solution ..............................................................................................8 Advantage: Reduces Many Memory Accesses to Zero Wait States.......................8 Disadvantage: Memory Accesses Still Bound By Bus Speed..................................8 The 80486 Solution: Internal Code/Data Cache ..............................................................9 Faster Memory Accesses..............................................................................................9 Frees Up the Bus...........................................................................................................9 The Floating-Point Bottleneck ................................................................................................9 The 80386/80387 Solution ................................................................................................10 The 80486 Solution: Integrate the FPU............................................................................10 The 80486 Microarchitecture..................................................................................................10 The Intel Family of 486 Processors .......................................................................................12 80486 System Architecture vi Chapter 2: Functional Units The 80486 Functional Units....................................................................................................13 Introduction.......................................................................................................................13 The 80486 Bus Unit............................................................................................................15 The 80486 Cache Unit........................................................................................................15 The Instruction Pipeline/Decode Unit ...........................................................................16 Instruction Prefetch ....................................................................................................17 Two-Stage Instruction Decode..................................................................................18 Execution .....................................................................................................................18 Register Write-Back....................................................................................................18 The Control Unit ................................................................................................................18 The Floating-Point Unit ....................................................................................................19 The Datapath Unit .............................................................................................................19 The Memory Management Unit (MMU)........................................................................20 Chapter 3: The Hardware Interface Hardware Interface..................................................................................................................21 General ...............................................................................................................................21 Clock .........................................................................................................................................23 Address.....................................................................................................................................23 Data Bus....................................................................................................................................24 Data Bus Parity.........................................................................................................................25 Bus Cycle Definition...............................................................................................................26 Bus Cycle Control ....................................................................................................................27 Burst Control ............................................................................................................................28 Interrupts..................................................................................................................................28 Bus Arbitration........................................................................................................................29 Cache Invalidation ..................................................................................................................30 Cache Control ...........................................................................................................................30 Numeric Error Reporting........................................................................................................32 Bus Size Control.......................................................................................................................32 Address Mask...........................................................................................................................33 SL Technology.........................................................................................................................33 Boundary Scan Interface ........................................................................................................34 Upgrade Processor Support ...................................................................................................35 Chapter 4: The 486 Cache and Line Fill Operations The 486 Caching Solution ......................................................................................................37 The 486 Internal Cache......................................................................................................37 The Advantage of a Level 2 Cache..................................................................................38 The 486 with an L2 Look-Through Cache ...........................................................................38 Contents vii Handling of I/O Reads .....................................................................................................40 Handling of I/O Writes ....................................................................................................40 Handling of Memory Reads.............................................................................................40 Handling of Memory Writes ............................................................................................41 Handling of Memory Reads by Another Bus Master ...................................................41 When a Write-Through Policy is Used ....................................................................42 When a Write-Back Policy is Used...........................................................................42 Handling of Memory Writes by Another Bus Master ..................................................42 When a Write-Through Policy is Used ....................................................................43 When a Write-Back Policy is Used...........................................................................43 The Bus Snooping Process .....................................................................................................45 Summary of the L2 Look-Through Cache Designs ...........................................................45 The 486 with an L2 Look-Aside Cache ................................................................................46 Anatomy of a Memory Read..................................................................................................48 The Internal Cache's View of Main Memory .................................................................48 L1 Memory Read Request ................................................................................................49 The Structure of the L1 Cache Controller.......................................................................49 Set the Cache Stage............................................................................................................50 The Cache Look-Up...........................................................................................................52 The Bus Cycle Request ......................................................................................................52 Memory Subsystem Agrees to Perform a Line Fill .......................................................54 Cache Line Fill Defined.....................................................................................................55 Conversion to a Cache Line Fill Operation ....................................................................56 L2 Cache's Interpretation of the Memory Address .......................................................56 The L2 Cache Look-Up .....................................................................................................57 The Affect of the L2 Cache Read Miss on the Microprocessor ....................................57 Organization of the DRAM Main Memory....................................................................57 The Cache Line Fill Transfer Sequence...........................................................................58 The First Doubleword Is Read from DRAM Memory ..................................................59 First Doubleword Transferred to the L2 Cache and the 80486 Microprocessor .................................................................................................59 Memory Subsystem's Treatment of the Next Three Doubleword Addresses ....................................................................................................60 Transfer of the Second Doubleword to the Microprocessor ........................................60 Memory Subsystem Latching of the Third and Fourth Doublewords .......................61 Transfer of the Third Doubleword..................................................................................61 The Beginning of the End .................................................................................................62 Transfer of the Fourth and Final Doubleword...............................................................62 Internal Cache Update ......................................................................................................62 Summary of the Memory Read........................................................................................64 Burst Transfers from Four-Way Interleaved Memory ......................................................64 Burst Transfers from L2 Cache..............................................................................................66 80486 System Architecture viii The Interrupted Burst .............................................................................................................67 Cache Line Fill Without Bursting.........................................................................................69 Internal Cache Handling of Memory Writes......................................................................73 Invalidation Cycles (486 Cache Snooping) .........................................................................73 L1 and L2 Cache Control ........................................................................................................74 Chapter 5: Bus Transactions (Non-Cache) Overview of 486 Bus Cycles...................................................................................................77 Bus Cycle Definition...............................................................................................................78 Interrupt Acknowledge Bus Cycle .......................................................................................79 Special Cycles..........................................................................................................................79 Shutdown Special Cycle ...................................................................................................80 Flush Special Cycle............................................................................................................80 Halt Special Cycle..............................................................................................................80 Stop Grant Acknowledge .................................................................................................81 Write-Back Special Cycle ..................................................................................................81 Non-Burst Bus Cycles .............................................................................................................81 Transfers with 8-,16-, and 32-bit Devices ............................................................................82 Address Translation..........................................................................................................82 Data Bus Steering...............................................................................................................84 Non-Cacheable Burst Reads ..................................................................................................85 Non-Cacheable Burst Writes .................................................................................................87 Locked Transfers......................................................................................................................89 Pseudo-Locked Transfers .......................................................................................................89 Transactions and BOFF# (Bus Cycle Restart) .....................................................................90 The Bus Cycle State Machine................................................................................................91 I/O Recovery Time...................................................................................................................92 Write Buffers ............................................................................................................................93 General ...............................................................................................................................93 The Write Buffers and I/O Cycles...................................................................................94 Chapter 6: SL Technology Introduction to SL Technology Used in the 486 Processors.............................................95 System Management Mode (SMM) .....................................................................................96 System Management Memory (SMRAM)......................................................................98 The SMRAM Address Map.......................................................................................98 Initializing SMRAM.................................................................................................101 Changing the SMRAM Base Address....................................................................101 Entering SMM..................................................................................................................101 The System Asserts SMI ..........................................................................................101 Back-to-Back SMI Requests..............................................................................102 SMI and Cache Coherency...............................................................................102 Contents ix Pending Writes are Flushed to System Memory..................................................102 SMIACT# is Asserted (SMRAM Accessed)...........................................................103 Processor Saves Its State ..........................................................................................103 Auto-HALT Restart...........................................................................................105 SMM Revision Identifier ..................................................................................105 SMBASE Slot ......................................................................................................106 I/O Instruction Restart .....................................................................................106 The Processor Enters SMM .....................................................................................107 Address Space...........................................................................................................108 Exceptions and Interrupts .......................................................................................108 Executing the SMI Handler ............................................................................................109 Exiting SMM.....................................................................................................................109 Processor’s Response to RSM..................................................................................109 State Save Area Restored.........................................................................................110 Maintaining Cache Coherency When SMRAM is Cacheable.............................111 486 Clock Control...................................................................................................................111 The Stop Grant State........................................................................................................111 Stop Clock State ...............................................................................................................113 Auto-HALT Power Down ..............................................................................................113 Stop Clock Snoop State ...................................................................................................114 Chapter 7: Summary of Software Changes Changes to the Software Environment..............................................................................115 Instruction Set Enhancements.............................................................................................116 The Register Set .....................................................................................................................117 Base Architecture Registers............................................................................................117 The System-Level Registers............................................................................................119 Control Register 0 (CR0)..........................................................................................120 Cache Disable (CD) and Not Write-Through (NW) .....................................121 Alignment Mask (AM)......................................................................................121 Write-Protect (WP) ............................................................................................122 Numeric Exception (NE) ..................................................................................122 Control Register 2 (CR2)..........................................................................................122 Control Register 3 (CR3)..........................................................................................123 Control Register 4 (CR4)..........................................................................................123 Global Descriptor Table Register (GDTR).............................................................124 Interrupt Descriptor Table Register (IDTR) ..........................................................124 Task State Segment Register (TR)...........................................................................124 Local Descriptor Table Register (LDTR)................................................................124 Virtual Paging ..................................................................................................................125 The Floating-Point Registers ...................................................................................126 The Debug and Test Registers ................................................................................128 80486 System Architecture x Chapter 8: The 486SX and 487SX Processors Introduction to the 80486SX and 80487SX Processors.....................................................131 The 486SX Signal Interface ..................................................................................................132 Register Differences..............................................................................................................132 Chapter 9: The 486DX2 and 486SX2 Processors The Clock Doubler Processors ............................................................................................135 Chapter 10: The Write Back Enhanced 486DX2 Introduction to the Write Back Enhanced 486DX2 ..........................................................137 Advantage of the Write-Back Policy ..................................................................................138 The Write-Through Policy..............................................................................................138 The Write-Back Policy.....................................................................................................139 Signal Interface ......................................................................................................................139 New Signals......................................................................................................................139 Existing Signals with Modified Functionality..............................................................141 The MESI Model....................................................................................................................141 Write Back Enhanced 486DX2 System without an L2 Cache.........................................144 Cache Line Fill..................................................................................................................144 Bus Master Read — Processor Snoop ...........................................................................146 Bus Master Write — Processor Snoop ..........................................................................148 Write Back Enhanced 486DX2 System with an L2 Cache...............................................150 The L2 Cache with a Write-Through Policy.................................................................151 The L2 Cache with a Write-Back Policy........................................................................152 Snoop Cycle During Cache Line Fill .............................................................................152 Special Cycles.........................................................................................................................155 Clock Control.........................................................................................................................156 Chapter 11: The 486DX4 Processor Primary Feature of the 486DX4 Processor .........................................................................159 Clock Multiplier ....................................................................................................................159 16KB Internal Cache..............................................................................................................160 5vdc Tolerant Design ............................................................................................................162 Glossary..................................................................................................................................165 Index........................................................................................................................................183 Figures xi Figure 1-1. Subsystems Integrated into the 80486 ................................................................ 11 Figure 2-1. 80486 Microarchitecture ....................................................................................... 14 Figure 2-2. The Elements Comprising the 80486 Bus Unit .................................................. 16 Figure 2-3. 80486 Instruction Pipeline.................................................................................... 17 Figure 3-1. 80486 Pin Designations......................................................................................... 22 Figure 4-1. The 486 Processor with an L2 Look-Through cache........................................ 39 Figure 4-2. The 80486 with a Look-Aside External Cache................................................... 47 Figure 4-3. The Structure of the L1 Cache ............................................................................. 51 Figure 4-4. Internal Cache Interpretation of the Memory Address.................................... 52 Figure 4-5. Memory Address at the Start of the Bus Cycle ................................................. 53 Figure 4-6. Cache Line Fill with Bursting .............................................................................. 54 Figure 4-7. 64-Bit Interleaved Memory Architecture ........................................................... 58 Figure 4-8. The LRU Algorithm .............................................................................................. 63 Figure 4-9. 4-way Interleaved Memory Designed to Support Burst Transfers. ............... 65 Figure 4-10. Burst Timing from 4-way Interleaved Memory............................................. 66 Figure 4-11. Burst Timing from L2 Cache.............................................................................. 67 Figure 4-12. The Interrupted Burst......................................................................................... 69 Figure 4-13. Non-Burst Cache Line Fill.................................................................................. 72 Figure 4-14. Cache Invalidation Cycle ................................................................................... 74 Figure 5-1. Example of Non-Burst Cycle Timing.................................................................. 82 Figure 5-2. Address Translation for 8, 16, and 32-bit Devices ............................................ 83 Figure 5-3. System Logic Used to Perform Data Bus Steering............................................ 84 Figure 5-4. Non-Cacheable Burst Read Bus Cycle................................................................ 86 Figure 5-5. Non-Cacheable Burst Write Bus Cycle............................................................... 88 Figure 5-6. 80486 Bus Cycle States.......................................................................................... 91 Figure 6-1. Address Space Available to Processor when Operating in Different Modes................................................................................................. 97 Figure 6-2. Sample Layout of SMM Memory........................................................................ 99 Figure 6-3. Typical PC Memory Map (SMM Disabled versus SMM Enabled)................. 100 Figure 6-4. The Processor’s SMM State-Save Map ............................................................... 104 Figure 6-5. SMM Revision Identifier Definition ................................................................... 105 Figure 6-6. Stop Clock State Diagram .................................................................................... 112 Figure 7-1. The BSWAP Instruction........................................................................................ 117 Figure 7-2. 80486 Base Architecture Registers ...................................................................... 118 Figure 7-3. 486 EFlags Register Definition............................................................................. 119 Figure 7-4. 80486 System Registers......................................................................................... 120 Figure 7-5. Bit definition for CR0............................................................................................ 121 Figure 7-6. Format of CR3........................................................................................................ 123 Figure 7-7. Format of CR4........................................................................................................ 124 Figure 7-8. The 80486 Floating-Point Registers..................................................................... 128 Figure 7-9. The 80486 Debug and Test Registers .................................................................. 129 80486 System Architecture xii Figure 10-1. Example of System with Write Back Enhanced 486DX2 (no L2 Cache)............................................................................................................................ 145 Figure 10-2. Example Cache Line Fill — Write-Back Mode Enabled................................. 146 Figure 10-3. External Snoop Performed by Enhanced Write Back 486DX2 Processor ..................................................................................................................... 149 Figure 10-4. Write Back Enhanced 486 with Look-Through L2 Cache.............................. 150 Figure 10-5. Cache Line Fill with External Snoop ................................................................ 154 Figure 10-6. Stop Clock State Machine for Enhanced Bus Mode ....................................... 156 Figure 11-1. Organization of the 486DX4 Internal Cache.................................................... 161