Bus-Based Computer Systems Flashcards
What is the function of a CPU bus?
allows memory, CPU, and devices to communicate
What is a bus?
a set of wires/communication protocol
What determines how devices communicate?
bus protocol
What happens to devices on the bus?
they go through a sequence of states
What are protocols specified by?
state machines
How many state machines per actor are there in the protocol?
one
Can bus protocols contain asynchronous logic?
yes
What are the steps of the four-cycle handshake?
dev 1 raises enquiry, dev 2 responds with acknowledgment, device 2 lowers acknowledgment once it is done, device 1 lowers enquiry
What provides synchronization in microprocessor busses?
the clock
R/W bit is true during what action?
reading
What is the name of the signal for when the data is ready?
Data ready bit
Address and data are how many lines?
a or n bit bundles of address and data lines (respectively)
How is the behavior of a bus most often specified?
a timing diagram
What are shown on timing diagrams to make sure signals go to the proper values at the proper times?
timing constraints
What must be provided when the data is ready?
Acknowledge
What does the data ready signal allow for?
the bus to be connected to devices that are slower than the bus
What is the wait state?
the cycles between the minimum time at which data can be asserted and when it is actually asserted
What is the wait state used for?
to connect slow, inexpensive memories to buses
What is DMA?
direct memory access which performs data transfers without executing instructions
What device sets up a data transfer?
the CPU
What does the DMA engine do during a data transfer?
fetches and writes data
What is a bus master by default?
CPU, sets up transfers
What must the DMA do to perform its work?
become the bus master
What is the bus mastership protocol?
bus request (DMA controller->CPU) then bus grant (CPU->DMA controller)