Direct memory access (DMA) transmitter

A direct memory access (DMA) transmitter adapted to transfer data from a random access memory to an output, such data having been generated by a central processing unit coupled to a bus, such generated data being initially stored in a local cache memory connected to the central processing unit, such transmitter being operative in response to a transmit write enable signal produced from an external source requesting the data, such external source also sending to the DMA transmitter a random access memory address, such DMA transmitter being coupled to the bus, such transmitter comprising:

• (a) an address register;
• (b) a data register;
• and
• (c) a transmitter state machine, a transmitter state machine placing an address in the address register in response to the write enable signal;
• (d) wherein requested data at such address is read from the random access memory then transferred for storage in the data register;
• (e) wherein the central processing unit also sends a control signal to the transmit state machine, such control signal indicating to the transmit state machine whether the read data is a most recent copy of the requested data in random access memory or whether the most recent copy of the requested data is still resident in the local cache memory;
• and
• (f) wherein, in response to the control signal, if the most recent data is in the local cache memory, the transmit state machine inhibits the data that was read from random access memory and now stored in data register from passing to the transmitter output, and the transmit state machine then performs a second data transfer request at the same address, the second requested data being transferred from the local cache memory to the random access memory, and the transmit state machine reads the second requested data from the random access memory, such second requested data being the most recent data available in the random access memory, the transmit state machine then storing such second requested data into the data register, such stored second requested data then being transferred to the transmitter output.

A method for performing a direct memory access (DMA) transfers of data from a random access memory to an output, comprising:

• generating the data in a central processing unit coupled to a bus, such generated data being initially stored in a local cache memory connected to the central processing unit;
• receiving from an external source, a random access memory address for storage in an address register and a transmit write enable signal for enabling storage of an address in the address register;
• sending a bus request signal to a bus arbiter connected to the bus;
• performing a bus arbitration and, when appropriate, having such arbiter grant access to the bus;
• when granted such bus, placing the address currently in the address register on an address portion of the bus for the random access memory along with appropriate read control signals on a control signal portion of the bus, the control signal indicating whether the read data is a most recent copy of the requested data in random access memory or whether the most recent copy of the requested data is still resident in the local cache memory;
• reading the data at the address on the bus from the random access memory;
• passing, via a data portion of the bus, the read data to the data register;
• if the control signal indicates that the most recent data is in the local cache memory:
• the data that was just read from random access memory, and which has been stored in data register, is inhibited from passing to the transmitter output;
• a second data transfer request at the same address location is made, the second requested data being transferred by the central processing unit from the local cache memory to the random access memory;
• performing a re-arbitration for the bus;
• and
• after the bus is granted, the second requested data is read from the random access memory, such second requested data being the most recent data available in the random access memory;
• loading such second requested data into the data memory in response to the assertion of the signal produced by the transmit state machine, such stored second requested data then being transferred to the transmitter output.