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The Intel x86 Hardware Organization

The Intel x86 Hardware Organization. AX. CS. BX. DS. CX. SS. DX. SS. IP. BP. DI. SI. SP. ALU. Flags Reg. The 8086 Processor. Execution Unit. Bus Interface Unit. Bus Control Unit. System Bus. Internal Bus. Control Unit. Instruction Queue. 64K base memory ROM.

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The Intel x86 Hardware Organization

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  1. The Intel x86 Hardware Organization UHD:CS2401: A. Berrached

  2. AX CS BX DS CX SS DX SS IP BP DI SI SP ALU Flags Reg. The 8086 Processor Execution Unit Bus Interface Unit Bus Control Unit System Bus Internal Bus Control Unit Instruction Queue UHD:CS2401: A. Berrached

  3. 64K base memory ROM 192k memory expansion area (ROM) Upper Memory 128k video display area (RAM) Conventional Memory 640K memory (RAM) Internal Memory • 256 KB of ROM and 768 KB of RAM Starting Address 960K F0000 768K C0000 640K A0000 9FFFF 0 00000 UHD:CS2401: A. Berrached

  4. The Internal memory of the 8086 can be up to 1 M bytes large. • How many bits are needed to express memory addresses ? Answer: 1 Meg = 220 => 20-bit addresses UHD:CS2401: A. Berrached

  5. The 8086 Register Set • Segment Registers: CS, DS, SS, ES • General-Purpose Registers: AX, BX, CX, DX • Pointer Registers SP, BP • Index Registers: SI, DI • Instruction Pointer IP • A Flags Register For the 8086, all registers are 16-bits large Bits in registers are numbered right to left, starting from 0. UHD:CS2401: A. Berrached

  6. Registers High-order byte Low-order byte Example: Store 17F6H in register 1 7 F 6 UHD:CS2401: A. Berrached

  7. AX: AH AL General-Purpose Registers • AX, BX, CX, DX all 16-bits large AX: Accumulator register BX: Base register CX: Count register DX: Data register • You can address them as one word or 1 byte. • E.g.: • All other registers must be accessed as the 16-bit. UHD:CS2401: A. Berrached

  8. Using General-Purpose Registers MOV instruction: copies data from one location into another. MOV Destination, Source ; Copy source to dest. E.g. MOV DX, CX Copies contents of register CX into register DX. • The MOV instruction does not effect the source operand • But Source and destination must be of same size. MOV AH, BH; MOV AH, BX ; Can’t do UHD:CS2401: A. Berrached

  9. Unising General-Purpose Registers MOV AX, 3D2BH ; load 3D2BH into AX MOV BH, 42H ; load 42H in high byte of BX ADD AL, 32H ; add 32H to AL Note: 80386 and later processor have 32-bit extended versions: EAX, EBX, ECX, and EDX MOV EAX, FF22AA00H ; (386 & up) UHD:CS2401: A. Berrached

  10. Program Segments • A segment is an area in memory that includes up to 64K bytes and begins on a paragraph boundary; ie. on an address evenly divisible by 16. Note: When an address is evenly divisible by 16 (i.e. 10H), its least significant four bits are 0s. => How many bits are needed for segment starting address? 16-bits • A program usually has one code segment (for the code), a data segment (for the data used by the program) and a stack segment. UHD:CS2401: A. Berrached

  11. Program Segments • The three types of segments are • code segment: contains machine instructions that make up the program. The starting address of the code segment is stored in the Code Segment (CS) register • data segment: contains the data used by the program. The address of the data segment is stored in the Data Segment (DS) register. • stack segment: the stack is used for temporary storage to save data temporarily or during subroutine calls. The address of the stack register is stored in the Stack Segment (SS) register. UHD:CS2401: A. Berrached

  12. Segment Registers • So when writing a program you always need to: • load CS with the starting address of the code segment • load DS with the starting address of the data segment • load SS with the starting address of the stack segment • All addressing within the program is done relative to segment registers. UHD:CS2401: A. Berrached

  13. Relative Addressing--Offset Address • The distance in bytes from a segment address to another location within the segment is called the offset address. • the first byte of a segment is at offset address 0 • the second byte is at offset 1 • An offset can range from 0 to 65,535 (0000-FFFF) => offsets are expressed as 16-bit unsigned binary numbers. UHD:CS2401: A. Berrached

  14. Relative Addressing--Logical Address • Generally, memory locations are expressed as Segment_Register:Offset meaning that the address is “Offset” bytes from the starting address of the segment. This is what is called the logical address. Example: DS:17H UHD:CS2401: A. Berrached

  15. Relative Addressing--Physical Address • The physical address is the actual RAM or ROM address. In the 8086, it is a 20 bit address. • How to compute the physical address of Seg_Reg:Offset ? • Shift the Seg_Reg left one Hex digit (pad with 0 at the right). • Add the Offset. UHD:CS2401: A. Berrached

  16. 0 2 7 A Relative Addressing: Example • Data segment starting address is 027A0H => DS register • Because segment starting addresses must be evenly divisible by 16, the leftmost hexadecimal digit is always 0. => The leftmost hex digit of a segment starting address is not stored in the segment register. UHD:CS2401: A. Berrached

  17. Assume we need to access data stored in the 24th byte of data segment => offset = 17H Logical address: DS:17H Physical address: Start with DS: 027A Shift left DS: 027A0 Add offset + 17 -------------- 027B7 H UHD:CS2401: A. Berrached

  18. Pointer Registers • Instruction Pointer (IP) register • Stack Pointer (SP) register • Base Pointer (BP) register UHD:CS2401: A. Berrached

  19. Instruction Pointer • The IP register always Points to the next instruction to be executed. I.e. its contains the offset address of the next instruction that is to execute. • The address of the next instruction to be executed is formed by adding the content of IP to that of CS. CS:IP The IP register is always associated with the CS register. UHD:CS2401: A. Berrached

  20. Stack Pointer register • The SP contains the offset address of the top of the stack • The SP register is always associated with the SS (Stack Segment) register SS:SP UHD:CS2401: A. Berrached

  21. Base Pointer register • The BP register is used to facilitate referencing parameter (on function calls) • The BP register is also associated with the SS register. UHD:CS2401: A. Berrached

  22. Flags Register • A 16-bit register • 80386 and later processor have a 32-bit flags register • Each bit indicates a certain status: example: Overflow, Carry, Interrupt • Many instructions change the status of the flags • Some instruction test for certain bits of the flag register before taking action • example: JO ; jump if overflow bit is set UHD:CS2401: A. Berrached

  23. Addressing of operation and data • An instruction consists of • at least one operation (e.g., ADD, MOVE) • zero, one or more operands (to reference the data) • Generally, the first operand is the destination example: MOV AX, 25 ; Immediate operand MOV BX, AX ; register to register UHD:CS2401: A. Berrached

  24. Addressing of operation and data • More examples: MOV AL, [0034] ; indexed addressing • Physical address of operand =DS:34H => the [ ] indicate that the physical address of the operand is computed by taking what’s between the [ ] as an offset and adding it to DS UHD:CS2401: A. Berrached

  25. Addressing … - Cont. • An instruction may access more than one byte at a time e.g., MOV AX, 35F3H MOV [1500], AX This instruction will copy the contents of AX (i.e. 35F3H) in two 2 memory locations starting at address DS:1500. How are the two bytes arranged? Little endian convention: low order byte in low order memory address UHD:CS2401: A. Berrached

  26. Little endian convention Assume DS = 3146[0]H => DS:1500 = 32960H Memory …. …. F3 35 32960 32961 AX: 35 F3 AH AL UHD:CS2401: A. Berrached

  27. Instruction Operands Some More Examples: X1 DW 0 ; Define X1 as a word ….. MOV CX, X1 ; Move X1 to CX MOV CX, 25 ; Move 25 to CX MOV CX, DX ; Move content of DX to CX MOV CX, [DX] ; Indexed addressing => to get actual address of operand: use content of DX as an offset and add it to DS => actual address = DS + DX UHD:CS2401: A. Berrached

  28. Debug program • A debugger program for x86 Assembly • used to: • enter machine code • enter assembly code • write small programs (.COM files) • debug assembly programs UHD:CS2401: A. Berrached

  29. DEBUG Commands • A: Assemble assembler instructions into machine code • D: Display contents of memory at a specific address • E: Enter data/instruction into memory • G: (Go) run the program • T: Trace • P: Proceed: execute a set of instruction • R: display contents of Registers • W: Write (I.e. save) a program into disk • Q: Quit See Appendix E of Text book. UHD:CS2401: A. Berrached

  30. DEBUG • Few Basic Rules • case insensitive • uses the colon format (e.g., segment:offset) • uses hexadecimal notation • use a space to separate parameters UHD:CS2401: A. Berrached

  31. Debug - Cont. • Examples: • D DS:200 <RTN> • E CS:100 B8 23 01 01 05 25 00 <RTN> • R <RTN> • AX= BX= ………….. • ………….. • nnnn:0100 B82301 MOV AX, 0123 UHD:CS2401: A. Berrached

  32. D: Display Memory Content • Address | Hex representation | ASCII xxxx:xx10 xx…....xx-xx..…..xx x…….x xxxx:xx20 xx…....xx-xx..…..xx x…….x xxxx:xx30 xx…....xx-xx..…..xx x…….x …... xxxx:xx80 xx…....xx-xx..…..xx x…….x UHD:CS2401: A. Berrached

  33. Machine Language Example Machine LangAssembly Lang B82301 MOV AX, 0123 052500 ADD AX, 0025 8BCB MOV CX, BX 90 NOP UHD:CS2401: A. Berrached

  34. Debug Operations • Keying in program instructions • Executing program instructions • Displaying memory contents • Correcting an entry UHD:CS2401: A. Berrached

  35. ML Example: Defined data E CS:100 A1 00 02 03 06 02 02 ……. E DS:0200 23 01 25 00 00 00 ……. To view code: D CS:100,105 <RTN> To view data: D DS:200,205 <RTN> UHD:CS2401: A. Berrached

  36. An assembly Language Example • Assemble Command: A 100 <RTN> MOV CL, 42 MOV DL,2A ADD CL, DL NOP UHD:CS2401: A. Berrached

  37. An assembly .. - Cont. • Unassemble command U 100,106 <RTN> xxxx:0100 B142 MOV CL, 42 xxxx:0102 B22A MOV DL, 2A xxxx:0104 00D1 ADD CL,DL xxxx:0106 90 NOP UHD:CS2401: A. Berrached

  38. More on Debug • If you are executing a program again within the DEBUG make sure to reset IP register to 100. • R IP : 100 • Saving a program from within debug • use A or E to key in the code • N filename.COM • clear BX • replace CX with the size of the program • W UHD:CS2401: A. Berrached

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