General AGA Information
Some Amigas, such as the A4000 and A1200, are built around the
AGA custom chip set, which supersedes the earlier ECS and original chip sets in graphics capability.
The main graphics features of AGA Amigas include:
- All color display modes can display up to 256 colors from a palette
of 16.8 million (up from a maximum of 64 colors from a 4096-color palette on earlier Amigas).
- The new HAM-8 mode, available in any display resolution, can
display over 256,000 colors at once, from the 16.8 million-color palette (up from a maximum of 4096 colors using
the earlier HAM mode).
- The Mode Promotion feature uses scan-doubling to turn the standard
15 kHz screens used by most Amiga applications to a stable, non-flickering VGA-quality display. Earlier Amigas
required additional hardware to eliminate interlace flicker.
- Sprites, including the mouse pointers, can now be high-resolution
and can be up to four times larger.
- New monitor driver files allow the Amiga to use additional screen
resolutions and adapt to a wider variety of monitors.
- An autoscrolling A2024 monitor screen larger than the display
can be defined.
- AGA retains virtually complete compatibility with software developed
for ECS and earlier Amigas.
Tips for Working with AGA
To get the most from your AGA Amiga, we recommend:
- Using a multiscan monitor than can
sync down to 15 kHz if possible. A monitor that can display 15 kHz modes is required for initial setup
of the Amiga.
- Selecting the Mode Promotion gadget
in the IControl Preferences editor to enable Mode Promotion. Be sure the appropriate monitor driver
(DBLNTSC for NTSC Amigas, or DBLPAL for PAL Amigas) is in DEVS:Monitors.
- Placing only the monitor drivers for
the display modes you need in DEVS:Monitors. This reduces clutter in the ScreenMode Preferences editor.
- Turning off Mode Promotion when doing
video work or choosing an NTSC or PAL display mode if the application allows it. Video devices require
15 kHz output.
- Using the Amiga Early Startup Control options in case of software
compatibility problems. If an application appears not to work on an AGA/Release 3 Amiga, try first booting the
system with the Enhanced or Original custom chip set emulation available from the Display Options screen.
Mode Promotion
Mode Promotion is a feature of AGA Amigas intended to give you
the best possible display. When an application tries to open a normal NTSC or PAL (15 kHz) screen while Mode Promotion
is enabled, the screen is promoted to a solid, non-flickering 27 kHz display.
A non-interlaced screen (200/256 pixel high) is scan-doubled,
which fills in the blank scan lines; an interlaced screen (400/512 pixels high) is de-interlaced so that horizontal
lines do not flicker. This is similar to the operation of the Commodore Display Enhancer circuitry in the Amiga
3000. By default, Mode Promotion is off.
Not all display modes can be promoted. Only standard NTSC and
PAL screens are promoted. (Super-High Res modes are promoted by changing them to autoscrolling DBL High Res screens.)
Certain screens are not promoted if the program that opens the screen does not allow it. Video applications in
particular may force a standard NTSC/PAL screen. Also, games and other applications that bypass the operating system
are not promoted.
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Note:
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The horizontal and vertical overscan available when using
the DBLNTSC and DBLPAL monitors is slightly less than that of the equivalent non-promoted screen.
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Enabling Mode Promotion
To enable promotion, make sure the Mode Promotion gadget in the
IControl Preferences editor is checked and that you select User or Save to exit the editor. Also, the DBLNTSC or
DBLPAL monitor driver must be in DEVS:Monitors and be activated. If you did not boot with a given monitor in DEVS:Monitors
and want to activate it, double-click on its icon after dragging it into DEVS:Monitors.
