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x86 Memory Management

x86 Memory Management. Reviewing Some Terms New Terms Translating Addresses Converting Logical to Linear Address Page Translation. Reviewing Some Terms.

kadeem-cooke
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x86 Memory Management

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  1. x86 Memory Management • Reviewing Some Terms • New Terms • Translating Addresses • Converting Logical to Linear Address • Page Translation Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.

  2. Reviewing Some Terms • Multitasking permits multiple programs (or tasks) to run at the same time. The processor divides up its time between all of the running programs. • Segments are variable-sized areas of memory used by a program containing either code or data. • Segmentation provides a way to isolate memory segments from each other. This permits multiple programs to run simultaneously without interfering with each other. • A segment descriptor is a 64-bit value that identifies and describes a single memory segment: it contains information about the segment’s base address, access rights, size limit, type, and usage. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.

  3. New Terms • A segment selector is a 16-bit value stored in a segment register (CS, DS, SS, ES, FS, or GS). • provides an indirect reference to a memory segment • A logical address is a combination of a segment selector and a 32-bit offset. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.

  4. Translating Addresses • The x86 processor uses a one- or two-step process to convert a variable's logical address into a unique memory location. • The first step combines a segment value with a variable’s offset to create a linear address. • The second optional step, called page translation, converts a linear address to a physical address. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.

  5. Converting Logical to Linear Address The segment selector points to a segment descriptor, which contains the base address of a memory segment. The 32-bit offset from the logical address is added to the segment’s base address, generating a 32-bit linear address. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.

  6. Indexing into a Descriptor Table Each segment descriptor indexes into the program's local descriptor table (LDT). Each table entry is mapped to a linear address: Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.

  7. Paging (1 of 2) • Paging makes it possible for a computer to run a combination of programs that would not otherwise fit into memory. • Only part of a program must be kept in memory, while the remaining parts are kept on disk. • The memory used by the program is divided into small units called pages. • As the program runs, the processor selectively unloads inactive pages from memory and loads other pages that are immediately required. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.

  8. Paging (2 of 2) • OS maintains page directory and page tables • Page translation: CPU converts the linear address into a physical address • Page fault: occurs when a needed page is not in memory, and the CPU interrupts the program • OS copies the page into memory, program resumes execution Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.

  9. MS-DOS and the IBM-PC • Real-Address Mode • MS-DOS Memory Organization • MS-DOS Memory Map • Redirecting Input-Output • Software Interrupts • INT Instruction • Interrupt Vectoring Process • Common Interrupts Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.

  10. Real-Address Mode • Real-address mode (16-bit mode) programs have the following characteristics: • Max 1 megabyte addressable RAM • Single tasking • No memory boundary protection • Offsets are 16 bits • IBM PC-DOS: first Real-address OS for IBM-PC • Has roots in Gary Kildall's highly successful Digital Research CP/M • Later renamed to MS-DOS, owned by Microsoft Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.

  11. MS-DOS Memory Organization • Interrupt Vector Table • BIOS & DOS data • Software BIOS • MS-DOS kernel • Resident command processor • Transient programs • Video graphics & text • Reserved (device controllers) • ROM BIOS Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.

  12. MS-DOS Memory Map Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.

  13. Redirecting Input-Output (1 of 2) • Input-output devices and files are interchangeable • Three primary types of I/O: • Standard input (console, keyboard) • Standard output (console, display) • Standard error (console, display) • Symbols borrowed from Unix: • < symbol: get input from • > symbol: send output to • | symbol: pipe output from one process to another • Predefined device names: • PRN, CON, LPT1, LPT2, NUL, COM1, COM2 Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.

  14. Redirecting Input-Output (2 of 2) • Standard input, standard output can both be redirected • Standard error cannot be redirected • Suppose we have created a program named myprog.exe that reads from standard input and writes to standard output. Following are MS-DOS commands that demonstrate various types of redirection: myprog < infile.txt myprog > outfile.txt myprog < infile.txt > outfile.txt Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.

  15. INT Instruction • The INT instruction executes a software interrupt. • The code that handles the interrupt is called an interrupt handler. • Syntax: INT number (number = 0..FFh) The Interrupt Vector Table (IVT) holds a 32-bit segment-offset address for each possible interrupt handler. Interrupt Service Routine (ISR) is another name for interrupt handler. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.

  16. Interrupt Vectoring Process Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.

  17. Common Interrupts • INT 10h Video Services • INT 16h Keyboard Services • INT 17h Printer Services • INT 1Ah Time of Day • INT 1Ch User Timer Interrupt • INT 21h MS-DOS Services Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.

  18. What's Next • MS-DOS and the IBM-PC • MS-DOS Function Calls (INT 21h) • Standard MS-DOS File I/O Services Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.

  19. MS-DOS Function Calls (INT 21h) • ASCII Control Characters • Selected Output Functions • Selected Input Functions • Example: String Encryption • Date/Time Functions Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.

  20. INT 4Ch: Terminate Process • Ends the current process (program), returns an optional 8-bit return code to the calling process. • A return code of 0 usually indicates successful completion. mov ah,4Ch ; terminate process mov al,0 ; return code int 21h ; Same as: .EXIT 0 Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.

  21. Selected Output Functions • ASCII control characters • 02h, 06h - Write character to standard output • 05h - Write character to default printer • 09h - Write string to standard output • 40h - Write string to file or device Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.

  22. ASCII Control Characters • 08h - Backspace (moves one column to the left) • 09h - Horizontal tab (skips forward n columns) • 0Ah - Line feed (moves to next output line) • 0Ch - Form feed (moves to next printer page) • 0Dh - Carriage return (moves to leftmost output column) • 1Bh - Escape character Many INT 21h functions act upon the following control characters: Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.

  23. Write a backspace to standard output: mov ah,06h mov dl,08h int 21h INT 21h Functions 02h and 06h: Write Character to Standard Output Write the letter 'A' to standard output: mov ah,02h mov dl,’A’ int 21h or: mov ah,2 Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.

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