EEL 4709C Prof. Watson Herman Group 4 Ali Alshamma, Derek Montgomery, David Ortiz 11/11/2008

 Addressing  X86 Segment Registers  Instruction Formats

 Immediate:  Define and use constraints or set initial values.  No need of memory.  Direct:  Requires only one memory.  Indirect:  Requires two memory references to fetch the operand.

 Register:  Limited address space.  Used heavily on instruction set.  Register Indirect:  Same as indirect addressing.  Displacement:  Instructions required to have two address fields.

 Stack:  Block of locations.  Last in first out (LIFO).

 X86 refers to the most successful instruction set architecture in the history of personal computing.  X86 represents various Intel chips with model numbers ending with the number 86 (i.e. Intel 80386).  Variable instruction length.  CISC and not CISC at the same time! ( uses basic Intel 8086 features which is a CISC).  16, 32, 64 bit registers.

 View memory as multiple address spaces (segments).  Memory accessed by a single index register.  Set access privilege (real or protected modes).  Fixed segment size in real mode (64 KB and 16 bit offset).  Variable segment size in protected mode.

 Real Mode:  20 bits segmented memory address space (1 MB).  No multitasking or memory protection at hardware level.  Protected Mode:  Expanded address space of 16 MB.  Provides multitasking.  Provides memory protection preventing programs from corrupting each other.  Segment registers to store an index to the segment in a table.

Real Mode Protected Mode

 Mostly used in microprogramming.  Hold segment address of various items  16 bits long  Set by special instructions or general register  Critical for good execution of a program in multi- segment programming.

 CS:  Holds code segment in which the program run.  Changing its value freezes the computer.  DS:  Holds the data segment in which the program accesses.  Changing its value gives wrong data.  ES, FS, GS:  Available for far pointer addressing (i.e. video memory addressing).  SS:  Holds the stack segment the program uses.  Sometimes contains the same value as DS.  Changing its value gives unpredictable data related results.

 Advantages:  Larger address space.  Better execution performance.  Compatible with most microprocessors.  Fast.  Disadvantages:  Expensive.  Harder to program.

 Defines the layout of the bits of an instruction in terms of its constituent fields.  Has an opcode and implicit and explicit zero or more operands.

 Most basic design issue.  Depends on:  Memory size  Memory organization  Bus structure  Processor complexity  Processor speed

 Programmers want:  More opcodes.  More operands.  More addressing modes for flexibility issues.  Increase in memory size to accommodate the increasing use of virtual memory.  Ex: 64-bit instruction vs. 32-bit instruction.

 More opcodes = more bits in the opcode field.  Reduce the number of bits available for addressing in a given length of an instruction format.

 To determine the use of addressing bits, one should know:  Number of addressing modes.  Number of operands.  Register vs. memory  Number of register sets.  Address range.  Address granularity.

 Simplest instruction design for a general purpose computer.  12 bit instructions operating on 12 bit words.  Has a single general purpose register called the accumulator.

 36 bit instructions length.  Has 36 bit word length.  9-bit opcodes.

 More flexible addressing.  Price depends on the complexity of the processor.

 Complex instruction set.  Complex addressing capability.

 True of False: Displacement addressing combines the capabilities of direct and register indirect addressing.  Which Addressing mode requires two memory references to fetch the operand?  The last in first out (LIFO) queue refers to which addressing mode?  Give and example of an x86-compatible chip that is widely spread nowadays?  How long are the segment registers in terms of bits?  What are the advantages of using segment registers?  What are the uses of the SS segment register in multi-segment programming?  What is the simplest general purpose computer in terms of instruction design?  What does an instruction format define?  What is the price to pay for using variable length instructions?

 Computer Organization & Architecture (course book).  EJ:en.wikipedia.org/wiki/X86+x86+segment+register s&hl=en&ct=clnk&cd=1&gl=us&client=firefox-a EJ:en.wikipedia.org/wiki/X86+x86+segment+register s&hl=en&ct=clnk&cd=1&gl=us&client=firefox-a  EJ: 6+segment+registers&hl=en&ct=clnk&cd=21&gl=us &client=firefox-a