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Intro to Amiga hardware

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                                  CHAPTER 1


The Amiga family of computers consists of several models, each of which
has been designed on the same premise to provide the user with a low cost
computer that features high cost performance. The Amiga does this through
the use of custom silicon hardware that yields advanced graphics and
sound features.

There are three distinct models that make up the Amiga computer family:
the A500, A1000, and A2000. Though the models differ in price and
features, they have a common hardware nucleus that makes them software
compatible with one another. This chapter describes the Amiga's hardware
components and gives a brief overview of its graphics and sound features.

                              - Introduction 1 -


These are the hardware components of the Amiga:

o Motorola MC68000 16/32 bit main processor. The Amiga also supports the
68010, 68020, and 68030 processors as an option.

o 512K bytes of intemal RAM, expandable to 1 MB on the A500 and A2000.

o 256K bytes of ROM containing a real time, multitasking operating system
with sound, graphics, and animation support routines.

o Built-in 3.5 inch double sided disk drive.

o Expansion disk port for connecting up to three additional disk drives,
which may be either 3.5 inch or 5.25 inch, double sided.

o Fully programmable RS-232-C serial port.

o Fully prograrnmable parallel porl.

o Two button opto-mechanical mouse.

o Two reconfigurable controller ports (for mice, joysticks, light pens,
paddles, or custom controllers).

o A professional keyboard with numeric keypad, 10 function keys, and cursor
keys. A variety of international keyboards are also supported.

o Ports for simultaneous composite video, and analog or digital RGB output.

o Ports for left and right stereo audio from four special purpose audio

o Expansion options that allow you to add RAM, additional disk drives
(floppy or hard), peripherals, or co-processors.

Thc Motorola 68000 is a 16/32 bit microprocessor. The system clock speed for
NTSC Amigas is 7.15909 megahertz (PAL 7.09379 MHz). These speeds may vary
when using an extemal system clock, such as from a genlock. The 68000 has
an address space of 16 megabytes. In the Amiga, thc 68000 can address over
8 megabytes of continuous random access memory (RAM).

                              - 2 Introduction -

In addition to the 68000, the Amiga contains special purpose hardware
known as the "custom chips" that greatly enhance system performance. The
term "custom chips" refers to the 3 intergrated circuits which were
designed specifically for the Amiga computer. These three custom chips
(called Agnus, Paula, and Denise) each contain the logic to handle a
specific set of tasks, such as video, sound, direct memory access (DMA),
or graphics.

Among other functions, the custom chips provide the following:

o Bitplane generated, high resolution graphics capable of supporting both PAL
and NTSC video standards.

   - On NTSC systems the Amiga typically produces a 320 by 200 non-interlaced
     or 320 by 400 interlaced display in 32 colors and a 640 by 200 non-
     interlaced or 640 by 400 interlaced display in 16 colors.

   - On PAL systems, thc Amiga typically produces a 320 by 256 non-interlaced
     or 320 by 512 interlaced display in 32 colors, and a 640 by 256 non-
     interlaced or 640 by 512 interlaced display in 16 colors.

Additional video modes allow for the display of up to 4,096 colors on
screen simultaneously (hold-and-modify) or provide for larger, higher
resolution displays (overscan).

o A custom display co-processor that allows changes to most of the special
purpose registers in synchronization with the position of the video beam.
This allows such special effects as mid-screen changes to the color
palette, splitting the screen into multiple horizontal slices each having
different video resolutions and color depths, beam synchronized interrupt
generation for the 68000 and more. The co-processor can trigger many
times per screen, in the middle of lines, and at the beginning or during
the blanking interval. The co-processor itself can directly affect most
of the registers in the other custom chips, freeing the 68000 for general
computing tasks.

o 32 system color registers, each of which contains a twelve bit number as
four bits of RED, four bits of GREEN, and four bits of BLUE intensity
information. This allows a system color palette of 4,096 different
choices of color for each register.

o Eight reusable 16 bit wide sprites with up to 15 color choices per
sprite pixel (when sprites arc paired). A sprite is an easily movable
graphics object whose display is entirely independent of the background
(called a playfield); sprites can be displayed over or under this
background. A sprite is 16 low resolution pixels wide and an arbitrary
number of lines tall. After producing the last line of a sprite on the
screen, a sprite DMA channel may be used to produce yet another sprite
image elsewhere on screen (with at least one horizontal line between each
reuse of a sprite processor). Thus, many small sprites can be produced by
simply reusing the sprite processors appropriately.

o Dynamically controllable inter-object priority, with collision
detection. This means that the system can dynamically control the video
priority between the sprite objects and the bitplane backgrounds
(playfields). You can control which object or objects appear over or
under the background at any time.

                              - Introduction 3 -

Additionally, you can use system hardware to detect collisions between
objects and have your program react to such collisions.

o Custom bit blitter used for high speed data movement, adaptable to
bitplane animation. The blitter has been designed to efficiently retrieve
data from up to three sources, combine the data in one of 256 different
possible ways, and optionally store the combined data in a destination
area. This is one of the situations where the 68000 gives up memory
cycles to a DMA channel that can do the job more efficiently (see below).
The bit blitter, in a special mode, draws pattemed lines into
rectangularly organized memory regions at a speed of about 1 million dots
per second; and it can efficiently handle area fill.

o Audio consisting of four digital channels with independently
programmable volume and sampling rate. The audio channels retrieve their
control and data via direct memory access. Once started, each channel can
automatically play a specified waveform without further processor
interaction. Two channels are directed into each of the two stereo audio
outputs. The audio channels may be linked together to provide amplitude or
frequency modulation or both forms of modulation simultaneously.

o DMA controlled floppy disk read and write on a full track basis. This
means that the built-in disk can read over 5600 bytes of data in a single
disk revolution (11 sectors of 512 bytes each).

The interal memory shared by the custom chips and the 68000 CPU is also
called "chip memory". The original custom chips in the Amiga were
designed to be able to physically access up to 512K bytes of shared
memory. The new version of the Agnus custom chip was created which allows
the graphics and audio hardware to access up to a full megabyte of

The Amiga 500 and 2000 models were designed to be able to accept the new
Agnus custom chip, called "Fat Agnus", due to its square shape. Hence,
the A500 and A2000 have allocated a chip memory space of 1 MB. This
entire 1 MB space is subject to thc arbitration logic that controls the
CPU and custom chip accesses. On the A1000, only the first 512K bytes of
memory space is shared, chip memory.

These custom chips and the 68000 share memory on a fully interleaved
basis. Since the 68000 only needs to access the memory bus during each
altemate clock cycle in order to run full speed, the rest of the time the
memory bus is free for other activities. The custom chips use the memory
bus during thcse free cycles, effectivcly allowing thc 68000 to run at
full rated spced most of the time. We say "most of the time" because
there are some occasions when the special purpose hardware steals memory
cycles from the 68000, but with good reason. Specifically, the
coprocessor and the data moving DMA channel called the blitter can each
steal time from the 68000 for jobs they can do bctter than the 68000.
Thus, the system DMA channels are designed with maximum performance in
mind. The job to be done is performcd by the most efficient hardware
element available. Even when such cycle stealing occurs, it only blocks
the 68000's access to the internal, shared memory. When using ROM or
extemal memory, the 68000 always runs at full speed.

                              - 4 Introduction -

Another primary feature of the Amiga hardware is the ability to
dynamically control which part of the chip memory is used for the
background display. audio, and sprites. The Amiga is not limited to a
small, specific area of RAM for a frame buffer. Instead, the system
allows display bitplanes, spritc processor control lists, coprocessor
instruction lists, or audio channel control lists to be located anywhere
within chip memory.

This same region of memory can be accessed by the bit blitter. This
means, for example, that the user can store partial images at scattered
areas of chip memory and use these images for animation effects by
rapidly replacing on screen material while saving and restoring
background images. In fact, the Amiga includes firmware support for
display definition and control as well as support for animated objects
embedded within playfields.

In addition to the connectors for monochrome composite, and analog or
digital RGB monitors, the Amiga can be expanded to include a VCR or
camera interface. This system is capable of synchronizing with an external
video source and replacing the system background color with the extemal
image. This allows development of fully integrated video images with
computer generated graphics. Laser disk input is accepted in the same

Floppy disk storage is provided by a built in, 3.5 inch floppy disk
drive. Disks are 80 track, double sided, and formatted as 11 sectors per
track, 512 bytes per sector (over 900,000 bytes per disk). The disk
controller can read and write 320/360K IBM PCTM (MS-DOSTM) fommatted 3.5
or 5.25 inch disks, and 640/720K IBM PC (MS-DOS) fommatted 3.5 inch
disks. Extemal 3.5 inch or 5.25 inch disk drives can be added to the
system through the expansion connector. Circuitry for some of the
peripherals resides on Paula. Other chips handle various signals not
specifically assigned to any of the custom chips, including modem
controls, disk status sensing, disk motor and stepping controls, ROM
enable, parallel input/output interface, and keyboard interface.

The Amiga includes a standard RS-232-C serial port for extemal serial
input/output devices.

A keyboard with numeric keypad, cursor controls and 10 function keys is
included in the base system. For maximum flexibility, both key-down and
key-up signals are sent. The Amiga also supports a variety of
intemational keyboards. Many other types of controllers can be attached
through thc two controller ports on the base unit. You can use a mouse,
joystick, keypad, track-ball, light pen, or steering wheel controller in
either of the controller ports.

                              - Introduction 5 -

New peripheral devices may be easily added to all Amiga models. These
devices are automatically recognized and used by system software through
a well defined, well documented linking procedure called AUTOCONFIG.

On the A500 and A1000 models, peripheral devices can be added to the
Amiga's 86 pin expansion connector, including additional extemal RAM.
Extra disk units may be added from a connector at the rear of the unit.

The A2000 model provides the user with the same features as the A500 or
A1000, but with the added convenicnce of simple and extensive
expandability. The 86 pin, external connector of the A1000 and A500 is
not extemally accessible on the A2000. Instead, the A2000 contains 7
internal slots that allow many types of expansion boards to be quickly
and easily added inside the machine. These expansion boards may contain
coprocessors, RAM expansion, hard disk controllers, video or I/O ports.
There is also room to mount both floppy and hard disks intemally. The
A2000 also supports the special Bridgeboard coprocessor card. This
provides a complete IBM PC on a card and allows the Amiga to run MS-DOS
compatible software, while simultaneously running native Amiga software.

                              - 6 Introduction -


The examples in this book all demonstrate direct manipulation of the
Amiga hardware. However, as a general rule, it is not permissible to
directly access the hardware in the Amiga unless your software either has
full control of the system, or has arbitrated via the OS for exclusive
access to the panicular parts of the hardware you wish to control.

Almost all of the hardware discussed in this manual, most notably the
Blitter, Copper, playfield, sprite, CIA, trackdisk, and system control
hardware, are in either exclusive or arbitrated use by portions of the
Amiga OS in any running Amiga system. Additional hardware, such as the
audio, parallel, and serial hardware, may be in use by applications which
have allocated their use through the system software.

Before attempting to directly manipulate any part of the hardware in the
Amiga's multitasking environment, your application must first be granted
exclusive access to that hardware by the operating system library,
device, or resource which arbitrates its ownership. The operating system
functions for requesting and receiving control of parts of the Amiga
hardware are varied and are not within the scope of this manual. Generally
such functions, when available, will be found in the library, device, or
resource which manages that portion of the Amiga hardware in the
multitasking environment. The following list will help you to find the
appropriate operating system functions or mechanisms which may exist for
arbitrated access to the hardware discussed in this manual.

     Copper, Playfield, Sprite, Blitter - graphics.library
     Audio - audio.device
     Trackdisk - trackdisk.device, disk.resource
     Serial - serial.device, misc.resource
     Parallel - parallel.device, cia.resource, misc.resource
     Gameport - input.device, gameport.device, potgo.resource
     Keyboard - input.device, keyboard.device
     System Control - graphics.library, exec.library (interrupts)

Most of the examples in this book use the hw examples.i file (see
Appendix J) to define the chip register names. Hw_examples.i uses the
system include file hardware/custom.i to define the chip structures and
relative addresses. The values defined in hardware/custom.i and how
examples.i are offsets from the base chip register address space. In
general, this base value is defined as _custom and is resolved during
linking from amiga.lib. (_ciaa and _ciab are also resolved in this way.)

Normally, the base address is loaded into an address register and the
offsets given by hardware/custom.i and hw_examples.i are then used to
address the correct register.

                              - Introduction 7 -

The offset values of the registers are the addresses that the Copper must
use to talk to the registers.

for example, in assembler:

          INCLUDE "exec/types.i"
          INCLUDE "hardware/custom.i"

          XREF custom                     ; External reference

  Start:  lea    _custom,a0               ; Use a0 as base register
          move.w #$7FFF,intena(a0)        ; Disable all interupts

In C, you would use the structure definitions in hardware/custom.h For

#lnclude        "exec/types.h"
#include        "hardware/custom.h"

extern  struct  Custom  custom;

/* You may need to define the above external as
**  extern struct Custom far custom;
**  Check you compiler manual.

custom.intena = 0x7FFF;         /* Disable all interupts */

The Amiga hardware include files are generally supplicd with your
compiler or assembler. Listings of lhc hardware include files may also be
found in the Addison-Wesley Amiga ROM Kemel Manual "Includes and
Autodocs". Generally, the include file label names are very similar to
the equivalent hardware register list names with the following typical

o Address registers which have low word and high word components are
generally listed as two word sized registers in the hardware register
list, with each register name containing either a suffix or embedded "L"
or "H" for low and high. The include file label for the same register
will generally treat the whole register as a longword (32 bit) register,
and therefore will not contain the "L" or "H" distinction.

o Related sequential registers which are given individual names with
number suffixes in the hardware register list, are generally referenced
from a single base register definition in the include files. For example,
the color registers in the hardware list (COLOR00, COLOR01, etc.) would
be referenced from the "color" label defined in "hardware/custom.i"
(color+0, color+2, etc.).

o Examples of how to define the correct register offset can be found in
the hw_examples.i file listed in Appendix J.

                              - 8 Introduction -


The Amiga is available in a variety of models and configurations, and is
further diversified by a wealth of add-on expansion peripherals and
processor replacements. In addition, even standard Amiga hardware such as
the keyboard and floppy disks, are supplied by a number of different
manufacturers and may vary subtly in both their timing and in their
ability to perform outside of their specified capabilities.

The Amiga operating system is designed to operate the Amiga hardware
within spec, adapt to different hardware and RAM configurations, and
generally provide upward compatibility with any future hardware upgrades
or "add ons" envisioned by the designers. For maximum upward
compatibility, it is strongly suggested that programmers deal with the
hardware through the commands and functions provided by the Amiga
operating system.

If you find it necessary to program the hardware directly, then it is
your responsibility to write code which will work properly on various
models and configurations. Be sure to properly request and gain control
of the hardware you are manipulating, and be especially careful in the
following areas:

Do not jump into ROM. Beware of any example code that calls routines in
the $F80000 to $FFFFFF range. These are ROM addresses and the ROM
routines WILL move with every OS revision. The only supported interface
to system ROM code is through the provided library, device, and resource

Do not modify or depend on the format of any private system structures.
This includes the poking of copper lists, memory lists, and library

Do not depend on any address containing any particular system structure
or type of memory. The system modules dynamically allocate their memory
space when they are initialized. The addresses of system structures and
buffers differ with every OS, every model, and every configuration, as
does the amount of free memory and system stack usage. Remember that all
data for direct custom chip access must be in CHIP RAM. This includes bit
images (bitplanes, sprites, etc), sound samples, trackdisk buffers, and
copper lists.

Do not write spurious data to, or interpret undefined data from currently
unused bits or addresses in the custom chip space. All undefined bits
must be set to zero for writes, and ignored on reads.

Do not write data past the current end of custom chip space. Custom chips
may be extended or enhaneed to provide additional registers, or to use
currently undefined bits in existing registers.

All custom chip registers are read only OR write only. Do not read write
only registers, and do not write to read only registers.

                              - Introduction 9 -

Do not read, write, or use any currently undefined address ranges. The
current and future usage of such areas is reserved by Commodore and is
definitely subject to change.

If you are using the system libraries, devices, and resources, you must
follow the defined interface. Assembler programmers (and compiler
writers) must enter functions through the liberary base jump tables, with
arguments passed as longs and library base address in A6. Results returned
in D0 must be tested, and the contents of D0-D1/A0-A1 must be assumed
gone after a system call.

The assembler TAS instruction should not be used in any Amiga program.
The TAS instruction assumes an indivisible read-modify-write but this can
be defeated by system DMA. Instead use BSET and BCLR. These instructions
perform a test and set operation which cannot be interrupted.

TAS is only needed for a multiple CPU system. On a single CPU system,
the BSET and BCLR instructions are identical to TAS, as the 68000 does
not interrupt instructions in the middle. BSET and BCLR first test, then
set bits.

Do not use assembler instructions which are privileged on any 68000
family processor, most notably MOVE SR,<ea> which is privileged on the
68010/20/30. Use the Exec function GetCC() instead of MOVE SR, or use the
appropriate non-privileged instruction as shown below:

            CPU        User Mode        Super Mode
           68000       MOVE SR,<ea>     MOVE SR,<ea>
         68010/20/30   MOVE CCR,<ea>    MOVE SR,<ea>

All addresses must be 32 bits. Do not use the upper 8 bits for other
data, and do not use signed variables or signed math for addresses. Do
not execute code on your stack or use self-modifying code since such code
can be defeated by the caching capabilities of some 68xxx processors. And
never use processor or clock speed dependent software loops for timing
delays. See Appendix F for information on using an 8520 timer for delays.

When strobing any register which responds to either a read or a write,
(for example copjmp2) be sure to use a MOVE.W #$00, not CLR.W. The CLR
instruction causes a read and a clear (two accesses) on a 68000, but only
a single access on 68020 and above. This will give different results on
different processors.

If you are programming at the hardware level, you must follow hardware
interfacing specifications. All hardware is NOT the same. Do not assume
that low level hacks for speed or copy protection will work on all
drives, or all keyboards, or all systems, or future systems. Test your
software on many different systems, with different processors, OS,
hardware, and RAM configurations.

                             - 10 Introduction -

      See FIGURE 1-1: Block Diagram for the Amiga Computer Family.

                             - 11 Introduction -