Editing Amiga Original Chip Set (section)

==Paula==
[[File:Custom Chip Paula 8364 R4 used in Amiga 1000.jpg|thumb|220px|Paula chip (MOS Technology 8364 R4) used in Amiga&nbsp;1000]]
[[File:CBM 8364R4 top metal.jpg|thumb|220px|Paula chip (MOS Technology 8364 R4) die]]
The Paula chip, designed by Glenn Keller, from [[MOS Technology]], is the [[interrupt controller]], but also includes logic for audio playback, floppy disk drive control, serial port [[input/output]] and mouse/joystick buttons two and three signals. The logic remained functionally identical across all Amiga models from Commodore.

===Audio===
Paula has four [[direct memory access|DMA]]-driven 8-bit [[pulse-code modulation|PCM]] sound channels. Two sound channels are mixed into the left audio output, and the other two are mixed into the right output, producing [[Stereophonic sound|stereo]] audio output. The only supported hardware sample format is signed linear 8-bit [[two's complement]]. Each sound channel has an independent frequency and a 6-bit volume control (64 levels). Internally, the audio hardware is implemented by four state machines, each having eight different states.

Additionally the hardware allows one channel in a channel pair to modulate the other channel's period or amplitude. It is rarely used on the Amiga due to both frequency and volume being controllable in better ways, but could be used to achieve different kinds of [[tremolo]] and [[vibrato]], and even rudimentary [[Fm synth|FM synthesis]] effects.

Audio may be output using two methods. Most often, DMA-driven audio is used. As explained in the discussion of Agnus, memory access is prioritized and one DMA slot per scan line is available for each of the four sound channels. On a regular NTSC or PAL display, DMA audio playback is limited to a maximum output rate of 28,867 values per channel (PAL: 28837) per second totaling 57674 (PAL: 57734) values per second on each stereo output. This rate can be increased with the ECS and AGA chipsets by using a video mode with higher [[horizontal scan rate]].

Alternately, Paula may signal the CPU to load a new sample into any of the four audio output buffers by generating an interrupt when a new sample is needed. This allows for output rates that exceed 57&nbsp;kHz per channel and increases the number of possible voices (simultaneous sounds) through software mixing.

The Amiga contains an analog [[low-pass filter]] ([[reconstruction filter]]) which is external to Paula. The filter is a 12&nbsp;dB/oct [[Butterworth filter|Butterworth]] low-pass filter at approximately 3.3&nbsp;kHz. The filter can only be applied globally to all four channels. In models after the Amiga&nbsp;1000 (excluding the very first revision of the Amiga&nbsp;500), the brightness of the power LED is used to indicate the status of the filter. The filter is active when the LED is at normal brightness, and deactivated when dimmed (on early Amiga&nbsp;500 models the LED went completely off). Models released before Amiga 1200 also have a static "tone knob" type low-pass filter that is enabled regardless of the optional "LED filter". This filter is a 6&nbsp;dB/oct low-pass filter with cutoff frequency at 4.5 or 5&nbsp;kHz.

A software technique was later developed which can play back 14-bit audio by combining two channels set at different volumes. This results in two 14-bit channels instead of four 8-bit channels. This is achieved by playing the high byte of a 16-bit sample at maximum volume, and the low byte at minimum volume (both ranges overlap, so the low byte needs to be shifted right two bits). The bit shift operation requires a small amount of CPU or blitter overhead, whereas conventional 8-bit playback is almost entirely DMA driven. This technique was incorporated into the retargetable audio subsystem [[AHI (Amiga)|AHI]], allowing compatible applications to use this mode transparently.

===Floppy disk controller===
The floppy controller is unusually flexible. It can read and write raw bit sequences directly from and to the disk via DMA or programmed I/O at 500 ([[double density]]) or 250&nbsp;kbit/s ([[single density]] or GCR). [[Modified Frequency Modulation|MFM]] or [[Group Coded Recording|GCR]] were the two most commonly used formats though in theory any [[run-length limited]] code could be used. It also provides a number of convenient features, such as sync-on-word (in MFM coding, $4489 is usually used as the [[sync word]]). MFM encoding/decoding is usually done with the blitter — one pass for decode, three passes for encode. Normally the entire track is read or written in one shot, rather than sector-by-sector; this made it possible to get rid of most of the inter-sector gaps that most floppy disk formats need to safely prevent the "bleeding" of a written sector into the previously-existing header of the next sector due to speed variations of the drive. If all sectors and their headers are always written in one go, such bleeding is only an issue at the end of the track (which still must not bleed back into its beginning), so that only one gap per track is needed. This way, for the native Amiga disk format, the raw storage capacity of 3.5&nbsp;inch DD disks was increased from the typical 720&nbsp;KB to 880&nbsp;KB, although the less-than-ideal [[Amiga Old File System|file system]] of the earlier Amiga models reduced this again to approximately 830&nbsp;KB of actual payload data.

In addition to the native 880&nbsp;KB 3.5-inch disk format, the controller can handle many foreign formats, such as:

* [[IBM PC]]
* [[Apple II]]
* [[Mac (computer)|Mac]] 800&nbsp;KB (requires a Mac drive)
* [[AMAX Mac emulator]] (a special floppy of only 200&nbsp;KB to exchange data between Amiga and Macintosh could be formatted by the Amiga, and it could be read and written by floppy drives of both systems)
* [[Commodore 1541]] (requires 5¼-inch drive slowed to 280&nbsp;rpm)
* [[Commodore 1581]] formatted {{frac|3|1|2}}-floppy for C64 and C128

The [[Amiga 3000]] introduced a special, dual-speed floppy drive that also allowed use of high density disks with double capacity without any change to Paula's floppy controller.

===Serial port===
The serial port is rudimentary, using [[programmed input/output]] only and lacking a [[FIFO (computing and electronics)|FIFO]] buffer. However, virtually any bit rate can be selected, including all standard rates, [[MIDI]] rate, as well as extremely high custom rates.