DMA Transfer Mode The transfer mode describes the

DMA

Transfer Mode • The transfer mode describes the way in which the data moves from the hard disk through the interface (IDE/ATA) and to the memory. • For example, it tells how fast data is transferred or what device is in charge of the transfer. • There are two basic categories – PIO (Programmed I/O) Mode • The processor micro-manages data transfer – DMA (Direct Memory Access) Mode • The processor delegates data transfer

Programmed I/O (PIO) Modes • In the PIO category, the processor controls the data transfer. • There are various PIO modes which differ mainly by speed. • Through the early to mid-90 s PIO was the standard way to transfer data to the hard disk. • The original ATA standards document defined the first three modes. • With ATA-2, two faster modes were introduced.

Standard PIO Modes 3. 3 MB/s = 3. 3 106 bytes / second = (2 bytes / 600 10 -9 s) Two bytes are transferred every 600 nanoseconds.

External rates • The PIO rates on the previous slide are external rates meaning that they reflect the rate that data in the hard disk’s buffer/cache can be transferred. • Recall that access times to locate and read from a random sector are of the order of milliseconds. – Reading a sector (512 bytes) in 20 ms would correspond to a rate of 25 KB/s. • If one were not buffering and transferring consecutive data, the PIO mode rates would be sufficient. • But we do transfer buffered data and the PIO transfer rates are considered prohibitively slow by today’s standards.

PIO is too, too slow • PIO is slow in two ways: – One does not achieve the same data transfer rates as with Ultra DMA, which is the standard transfer mode used for IDE/ATA today. – Because the processor controls the details of the transfer in PIO, the processor is distracted from performing other tasks. • Despite its slowness, PIO is still around because: – PIO is simple (built into the BIOS so it does not require drivers). – Backward compatibility – Can be used as a backup when something goes wrong with DMA.

DMA • The alternative to PIO is DMA, Direct Memory Access. • In DMA, a device transfers information to or from the memory directly rather than in a processorcontrolled fashion. • DMA has been around awhile but it was not always well supported early on. But speed requirements have made it preferred over PIO.

Various Modes • As with PIO, DMA has various modes differing mainly by speed. • DMA modes split into two categories: – Single-word modes which send one word (two bytes) at a time – Multiple-word modes which send several words in rapid succession (rather like the idea of bursting that accounts for improved memory speeds).

Assume it’s multiword • The single-word DMA modes are too slow, today it is understood that DMA is multiword DMA and the term is rarely mentioned and usually implied. • In fact, the single-word DMA modes were dropped from the standards with ATA-2. • Ultra DMA is multiword.

First Party “The party of the first part shall be known in this contract as the party of the first part”

First Party vs. Third Party • In Third-Party DMA there is a third device, the DMA controller mediating the transfer between the hard disk and memory. – Third party DMA is slow and old fashioned. • In First-Party DMA, a. k. a. bus mastering, the middle man is eliminated. The hard drive controls the transfer of data between itself and memory. – The device (hard drive in this case) takes control of (masters) the bus along which the information is sent.

Ultra DMA • DMA only became the norm with the introduction of Ultra DMA. – It just wasn’t well supported before. • DMA gained the advantage over PIO when Ultra DMA/33 doubled the interface transfer rate. – Support for it also improved. • Ultra DMA became an industry standard.

Ultra DMA uses DDR and CRC • One feature that made Ultra DMA “ultra” was that it transferred data on both the positive and negative edges of the clock. – Same idea as in DDR (Double Data Rate) memory • As Ultra DMA pushed the limit on transfer rate, it made the occurrence of errors somewhat more likely. Thus it introduced a CRC (Cyclic Redundancy Check) as part of the standard. – Recall CRC is error detection. If an error occurs in transmission the data is retransmitted.

Ultra DMA transfer rates DMA finally beats PIO Mode 4’s 16. 7 MB/s with the added bonus of freeing up the processor. The modes are usually named after their maximum transfer rate and the interface they use: Ultra ATA/100 instead of Ultra DMA/Mode 5.

New cable needed • The faster speeds did require a change in the cable used to connect the drive. The 80 -conductor ATA/EDE cable. • Color Code – Blue: connects to the host (motherboard or controller). – Gray: connects to the slave drive (if there is one) – Black: connects to the master drive

80 wires and 40 pins? ? (pre SATA) • Signals are varying currents. • Currents produce magnetic fields, varying currents produce varying magnetic fields. • Varying magnetic fields produce currents. • Oops, the current in wire 1 begins to change the current in wire 2. • There is interference (a. k. a. cross talk) • The extra wires shield the signal-carrying wires from each other.

Block Mode • Certain BIOSs allow for a Block Mode setting. Block Mode allows 16 or 32 sectors (512 bytes each) to be handled using a single interrupt (even if the processor is not running the show it needs to know something has happened).

References • PC Hardware in a Nutshell (Thompson and Thompson) • http: //www. pcguide. com • All-in-One A+ Certification, Meyers and Jernigan • http: //www. webopedia. com • http: //www. serialata. org CSIT 301 (Blum) 19