Computer Interfaces
The Computer Level Hierarchy
• Each virtual machine
layer is an abstraction of
the level below it.
• The machines at each
level execute their own
particular instructions,
calling upon machines at
lower levels to perform
tasks as required.
• Computer circuits
ultimately carry out the
work.
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The Computer Level Hierarchy
• Level 6: The User Level
– Program execution and user interface level.
– The level with which we are most familiar.
• Level 5: High-Level Language Level
– The level with which we interact when we write
programs in languages such as C, Pascal, Lisp, and
Java.
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The Computer Level Hierarchy
• Level 4: Assembly Language Level
– Acts upon assembly language produced from
Level 5, as well as instructions programmed
directly at this level.
• Level 3: System Software Level
– Controls executing processes on the system.
– Protects system resources.
– Assembly language instructions often pass
through Level 3 without modification.
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The Computer Level Hierarchy
• Level 2: Machine Level
– Also known as the Instruction Set Architecture
(ISA) Level.
– Consists of instructions that are particular to the
architecture of the machine.
– Programs written in machine language need no
compilers, interpreters, or assemblers.
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The Computer Level Hierarchy
• Level 1: Control Level
– A control unit decodes and executes instructions
and moves data through the system.
– Control units can be microprogrammed or
hardwired.
– A microprogram is a program written in a low-level
language that is implemented by the hardware.
– Hardwired control units consist of hardware that
directly executes machine instructions.
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The Computer Level Hierarchy
• Level 0: Digital Logic Level
– This level is where we find digital circuits (the
chips).
– Digital circuits consist of gates and wires.
– These components implement the mathematical
logic of all other levels.
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8086/88 Device Specifications
• Both are packaged in DIP (Dual In-Line Packages)
•
8086: 16-bit microprocessor with a 16-bit data bus
•
8088: 16-bit microprocessor with an 8-bit data bus
• Both are 5V parts (i.e. VDD is 5V)
•
8086: Draws a maximum supply current of 360mA
•
8086: Draws a maximum supply current of 340mA
•
80C86/80C88: CMOS version draws 10mA
with temp spec -40 to 225oF
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Input/Output current levels:
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TTL/CMOS Logic Levels
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8086/88 Pinout
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8086 Fully Buffered
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BUS Buffering and Latching
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Bus Latching and Buffering
•
•
•
•
•
•
Latches are used to de-multiplex the
address/data and address/status lines and
commonly have output buffers for driving
external loads.
• Buffers are used to drive external loads,
and to isolate component when disabled.
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Bus Latching and Buffering
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74LS244 3-STATE Buffer
• When enabled by the control line, output
follows input (buffered, pass-through).
• When disabled, output is a very high
impedance which prevents the output from
driving or loading connected circuits.
• When disabled, the outputs are said to be
floating.
• In effect, it is like a switch.
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74LS245 Bidirectional buffers
(transceivers)
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74LS373 Latch Latches (D-type flip-flops)
• When enable is high, Q follows D.
• When enable goes low, Q maintains
(latches) state of D.
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A fully buffered 8086
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BUS Timing
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BUS Timing
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“slots” on the motherboard
3 ISA
slots
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XT-CARD
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XT-bus
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XT-bus,
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