University:
California State University, Northridge
Course:
COMP 122/L | Computer Architecture and Assembly Language and Lab
Academic year:

2024
Views:

377

Pages:

39
Author:

Alonso Charles

 What are external devices?  Three categories: • Human readable • Machine readable • Communication  Keyboard/Monitor • The International Reference Alphabet (IRA) • IRA Control Characters  Disk Drive  Control & Timing  CPU Communication  Device Communication  Data Buffering  Error Detection  CPU checks I/O module device status  I/O module returns status  If ready, CPU requests data transfer  I/O module gets data from device  I/O module transfers data to CPU  Variations for output, DMA, etc.  Hide or reveal device properties to CPU  Support multiple or single device  Control device functions or leave for CPU  Also O/S decisions • e.g. Unix treats everything it can as a file  Programmed  Interrupt driven  Direct Memory Access (DMA)  CPU issues address • Identifies module  CPU issues command • Control - telling module what to do  e.g. spin up disk • Test - check status  e.g. power? Error? • Read/Write  Module transfers data via buffer from/to device  CPU issues address • Identifies module (& device if >1 per module)  CPU issues command • Control - telling module what to do  e.g. spin up disk • Test - check status  e.g. power? Error? • Read/Write  Module transfers data via buffer from/to device  Under programmed I/O data transfer is very like memory access (CPU viewpoint)  Each device given unique identifier  CPU commands contain identifier (address)  Memory mapped I/O • Devices and memory share an address space • I/O looks just like memory read/write • No special commands for I/O  Large selection of memory access commands available  Isolated I/O • Separate address spaces • Need I/O or memory select lines • Special commands for I/O  Limited set  Overcomes CPU waiting  No repeated CPU checking of device  I/O module interrupts when ready  CPU issues read command  I/O module gets data from peripheral whilst CPU does other work  I/O module interrupts CPU  CPU requests data  I/O module transfers data  How do you identify the module issuing the interrupt?  How do you deal with multiple interrupts? • i.e. an interrupt handler being interrupted    To allow the 80386 to handle a variety of devices, it is usually configured with and external interrupt arbiter. The 82C59A External devices are connected to the 82C59A which in turn is connected to the 80386 A single 82C59 A can handle up to eight modules, if control for more than eight modules is required a cascade arrangement can be used The 82C59A is responsible for the management of the interrupts  The 82C59A is programmable  The 80836 determines the priority scheme to be used by setting a control word in the 82C59A  Interrupt modes: Fully nested: the interrupt requests are ordered from 0 (IR0) through 7 (IR7) Rotating: In some applications a number of interrupts devices are of equal priority. In this mode a device, after being serviced, receives the lowest priority in the group Special mask: this allows the processor to inhibit interrupts from certain devices     The 82C55A is a single-chip general-purpose I/O module designed for use with the Intel 80386 processor The 24 I/O lines are programmable by the 80386 by means of the control register The 24 lines are divided in three 8-bit groups (A, B, C)  Each group can function as an 8 bit port, in addition group C is subdivided into 4 bit groups.  THE 82C55A includes an 8 bits bidirectional data bus (D0 through D7) used to transfer data to and from the I/O ports  The reset line is used to initialized the module  The control register is loaded by the processor to control the mode of operation and to define signals  Control signals serve two principal purposes handshaking and interrupt request. Handshaking is a simple timing mechanism     Because the 82C55A is programmable via the control register, it can be used to control a variety of simple peripheral devices The keyboard provides 8 bits of input Two of these bits SHIFT and CONTROL have special meaning to the keyboard handing program executing in the processor The display is also linked by an 8 bit data port and again two lines have special meanings to the display.  Interrupt driven and programmed I/O require active CPU intervention • Transfer rate is limited • CPU is tied up  DMA is the answer  Additional Module (hardware) on bus  DMA controller takes over from CPU for I/O  CPU tells DMA controller:- • Read/Write • Device address • Starting address of memory block for data • Amount of data to be transferred  CPU carries on with other work  DMA controller deals with transfer  DMA controller sends interrupt when finished  DMA controller takes over bus for a cycle  Transfer of one word of data  Not an interrupt • CPU does not switch context  CPU bus suspended just before it accesses • i.e. before an operand or data fetch or a data write  Slows down CPU but not as much as CPU doing transfer  Single Bus, Detached DMA controller  Each transfer uses bus twice • I/O to DMA then DMA to memory  CPU is suspended twice  Single Bus, Integrated DMA controller  Controller may support >1 device  Each transfer uses bus once • DMA to memory  CPU is suspended once  Separate I/O Bus  Bus supports all DMA enabled devices  Each transfer uses bus once • DMA to memory  CPU is suspended once     Interfaces to 80x86 family and DRAM When DMA module needs buses it sends HOLD signal to processor CPU responds HLDA (hold acknowledge) • DMA module can use buses 1. 2. 3. Device requests service of DMA by pulling DREQ (DMA request) high DMA puts high on HRQ (hold request), CPU finishes present bus cycle (not necessarily present instruction) and puts high on HDLA (hold acknowledge). HOLD remains active for duration of DMA DMA activates DACK (DMA acknowledge), telling device to start transfer DMA starts transfer by putting address of first byte on address bus and activating MEMR; it then activates IOW to write to peripheral. DMA decrements counter and increments address pointer. Repeat until count reaches zero DMA deactivates HRQ, giving bus back to CPU E.g. transfer data from memory to disk 4. 5. 6.  While DMA using buses processor idle  Processor using bus, DMA idle • Known as fly-by DMA controller  Data does not pass through and is not stored in DMA chip • DMA only between I/O port and memory • Not between two I/O ports or two memory locations  Can do memory to memory via register  8237 contains four DMA channels • Programmed independently • Any one active • Numbered 0, 1, 2, and 3

Chapter 7. Input/Output

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