The oldest method of transferring data over the IDE/ATA interface is through the use of
programmed I/O. This is a technique whereby the system CPU and support hardware
directly control the transfer of data between the system and the hard disk. There are
several different speeds of programmed I/O, which are of course called programmed
I/O modes, or more commonly, PIO modes.
Through the mid-1990s, programmed I/O was the only way that most systems ever accessed
IDE/ATA hard disks. Three lower-speed modes were defined as part of the original ATA standards document; two more were added as part of ATA-2 (as well as part of several unofficial
standards.) The table below shows the five different PIO modes, along with the cycle
time for each transfer and the corresponding throughput of the PIO mode:
PIO Mode |
Cycle Time
(nanoseconds) |
Maximum Transfer
Rate (MB/s) |
Defining Standard |
Mode 0 |
600 |
3.3 |
ATA |
Mode 1 |
383 |
5.2 |
ATA |
Mode 2 |
240 |
8.3 |
ATA |
Mode 3 |
180 |
11.1 |
ATA-2 |
Mode 4 |
120 |
16.7 |
ATA-2 |
A few things about this table bear mention. First of all, the PIO modes are defined in
terms of their cycle time, representing how many nanoseconds it takes for each transfer to
occur. The maximum transfer rate is the reciprocal of the cycle time, doubled because the
IDE/ATA interface is two bytes (16 bits) wide. Also, conspicuous by its absence from the table above is the so-called
"PIO mode 5", which does not exist and was never implemented in any IDE/ATA hard
disks. Apparently, at one point some discussion occurred about creating a faster PIO mode,
which was tentatively called "PIO mode 5". This mode was to support a transfer
rate of 22.2 MB/s, but it was never implemented (probably because the much faster 33 MB/s Ultra DMA mode 2 was on the horizon). Some motherboard
manufacturers made a point of providing early support for this proposed mode in their BIOS
setup programs, so you may occasionally see it mentioned.
Obviously, faster modes are better, because they mean a higher theoretical burst
transfer rate over the interface. This transfer rate represents the external data transfer rate for the drive.
Remember that this is the speed of the interface and not necessarily the sustained transfer rate of the drive itself,
which is almost always slower (and should be). Of course today all new drives have
sustained transfer rates well in excess of what even the fastest PIO mode can handle,
which is one reason why PIO has fallen out of favor. Also worth mention is that very old
systems using ISA hard disk
controllers cannot use even PIO modes 3 or 4, because their transfer rate exceeds the
capacity of the ISA bus!
As I mentioned, programmed I/O is performed by the system CPU; the system processor is
responsible for executing the instructions that transfer the data to and from the drive,
using special I/O locations. This technique works fine for slow devices like keyboards and
modems, but for performance components like hard disks it causes performance issues. Not
only does PIO involved a lot of wasteful overhead, the CPU is "distracted" from
its ordinary work whenever a hard disk read or write is needed. This means that using PIO
is ideally suited for lower-performance applications and single tasking. It also means
that the more data the system must transfer, the more the CPU gets bogged down. As hard
disk transfer rates continue to increase, the load on the CPU would have continued to
grow. This is the other key reason why PIO modes are no longer used on new systems, having
been replaced by DMA modes, and then later, Ultra DMA.
As discussed in detail here, each IDE channel supports
the use of two devices, designated as master and slave. Modern systems allow the use of
master and slave devices running at different PIO modes on the same channel; this is
called independent device timing and is a function of the
system chipset and BIOS. When this feature is not supported, both devices may be limited
to the slower of the two devices' maximum PIO mode, but this hasn't been a big issue since
the mid-to-late 1990s.
PIO modes do not require any special drivers under normal circumstances; support for
them is built into the system BIOS. This universal support, along with their conceptual
simplicity, is why they were traditionally the default way that most drives are used.
Today, however, PIO is just not up to handling modern drives, which use Ultra DMA to keep
the load on the CPU down and to allow access to Ultra DMA's much higher performance.
Support for PIO modes is still universal on almost all systems and drives made since the
mid-1990s, for backwards compatibility. It is used, for example, as a "last
resort" when driver or software issues cause problems with Ultra DMA accesses.
Next: Direct Memory Access (DMA) Modes and Bus Mastering DMA
