COT 4600 Operating Systems Spring 2011

1. COT 4600 Operating Systems Spring 2011 Dan C. Marinescu Office: HEC 304 Office hours: Tu-Th 5:00 – 6:00 PM

2. Last time: Binding and indirection Generic naming model Name mapping algorithms Comparing names; name discovery Practical design of naming scheme Contexts Name overloading Today: Computer system organization; operating systems The hardware layer Bus sharing Optimization, DMA The software layer – the file abstraction Case studies UNIX File System URLs Next time Modularity Lecture 9 – Thursday, February 10, 2011 Lecture 9

3. Computer System Organization • Operating Systems (OS)  software used to • Control the allocation of resources (hardware and software) • Support user applications • Sandwiched between the hardware layer and the application layer • OS-bypass: the OS does not hide completely the hardware from applications. It only hides dangerous functions such as • I/O operations • Management function • Names  modularization Lecture 9

4. Figure 2.16 from the textbook Lecture 9

5. The hardware layer • Modules representing each of the three abstractions (memory, interpreter, communication link) are interconnected by a bus. • The bus  a broadcast communication channel, each module hears every transmission. • Control lines • Data lines • Address lines • Each module • is identified by a unique address • has a bus interface • Modules other than processors need a controller. Lecture 9

6. Figure 2.17 from the textbook Lecture 9

7. Bus sharing and optimization • Communication  broadcast • Arbitration protocol  decide which module has the control of the bus. Supported by hardware: • a bus arbiter circuit • distributed among interfaces – each module has a priority • daisy chaining • Split-transaction  a module uses the arbitration protocol to acquire control of the bus • Optimization: • hide the latency of I/O devices • Channels  dedicated processors capable to execute a channel program (IBM) • DMA (Direct Memory Access) • Support transparent access to files: • Memory Mapped I/O Lecture 9

8. Optimization • Direct Memory Access (DMA): • supports direct communication between processor and memory; the processor provides the disk address of a block in memory where data is to be read into or written from. • hides the disk latency; it allows the processor to execute a process while data is transferred • Memory Mapped I/O: • LOAD and STORE instructions access the registers and buffers of an I/O module • bus addresses are assigned to control registers and buffers of the I/O module • the processor maps bus addresses to its own address space (registers) • Supports software functions such as UNIX mmap which map an entire file. • Swap area: disk image of the virtual memory of a process. Lecture 9

9. DMA Transfer Lecture 9 9

10. B. The software layer: the file abstraction • File: memory abstraction used by the application and OS layers • linear array of bits/bytes • properties: • durable  information will not be changed in time • has a name  • allows access to individual bits/bytes  has a cursor which defines the current position in the file. • The OS provides an API (Application Programming Interface) supporting a range of file manipulation operations. • A user must first OPEN a file before accessing it and CLOSE it after it has finished with it. This strategy: • allows different access rights (READ, WRITE, READ-WRITE) • coordinate concurrent access to the file • Some file systems • use OPEN and CLOSE to enforce before-or-after atomicity • support all-or-nothing atomicity e.g., ensure that if the system crashes before a CLOSE either all or none of WRITEs are carried out Lecture 9

11. Open and Read operations Lecture 9 11

12. Unix and Unix File System • Developed in early 1970s at Bell Labs for PDP computers made by Digital • Lineage  MULTICS developed at MIT in 1960s • Versions: • Berkeley Unix (BSD) • GNU/Linux • Darwin – part of MAC OS • Unix file system – hierarchical data organization: blocks  files  directories  file systems • the objects: • files – linear arrays of blocks of data; each file has a cursor giving the current position • directories – collections of files; tree structure • metadata - useful information about the file, not contained in the file (e.g., owner, access modes, last modified date, length, etc.) • supports: • creation, deletion, renaming of files and directories • reading data from and writing data to a file • reading and writing metadata describing a file Lecture 9

13. API for the Unix File System OPEN(name, flags, model)  connect to a file Open an existing file called name, or Create a new file with permissions set to mode if flags is set. Set the file pointer (cursor) to 0. Return the file descriptor (fd). CLOSE(fd)  disconnect from a file Delete file descriptor fd. READ(fd, buf,n)  read from file Read n bytes from file fd into buf; start at the current cursor position and update the file cursor (cursor = cursor + n). WRITE(fd, buf,n)  write to file Write n bytes to the file fd from buf; start at the current cursor position and update the file cursor (cursor = cursor + n). SEEK(fd, offset,whence)  move cursor of file Set the cursor position of file fd to offset from the position specified by whence (beginning, end, current position) Lecture 9

14. API for the Unix File System (cont’d) FSYNC(fd)  make all changes to file fd durable. STAT(name) read metadata CHMOD, CHOWN  change access mode/ownership RENAME(from_name,to_name)  change file name LINK(name, link_name) create a hard link UNLINK(name) remove name from directory SYMLINK(name, link_name) create a symbolic link MKDIR(name) create directory name RMDIR(name) delete directory name CHDIR(name)  change current directory to name CHROOT  Change the default root directory name MOUNT(name,device)  mount the file system name onto device UNMOUNT(name)  unmount file system name Lecture 9