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-   -   Amiga HAM (http://eab.abime.net/showthread.php?t=40616)

Si-Pie 10 November 2008 19:36

Amiga HAM
Someone tried telling me today that the Amigas HAM mode was a interrupt trick like the Atari ST used to display all colours on screen at once but im sure this is wrong? The HAM mode was a screen mode implemented by design wasnt it and not a interrupt trick? Anyone shed any light on this?

AmigaFriend 10 November 2008 19:45

As always on these days:


If anyone can provide more info...

Si-Pie 10 November 2008 19:54

yep had a read through that but just want to get my facts right before I speak to him again and wanted make sure I wasnt missing anything. Thanks for the link! More info would be great please ....

Charlie 10 November 2008 20:00

HAM mode is indeed a deliberate part of the Amiga Chipset design.

Interesting fact:
If memory serves Jay Miner (:bowdown) wasn't happy with HAM & was going to pull it from the original chipset design just before production but didn't because it would have left an untidy hole in the middle of the silicone - there being no time to go through another Rev...

Si-Pie 10 November 2008 20:03

Yeh thats what I thought so basically it was not a interrupt trick like that used on the Atari ST? Oh and thanks for that interesting fact and thats something I didnt know.

AmigaFriend 10 November 2008 20:56

Yes indeed!

Si-Pie on the bottom of wikipedia page you have more info about that.

Till reading today on this page I wasn't even aware of this:

SHAM (Sliced HAM)

Rhett Anderson developed the so called SHAM or SHM mode, which was a standard 32-color mode, but allowed each video scan line to have its own, independent 32-color palette. This was possible because of a special co-processor that could reprogram the color palette registers at the beginning of each scan line. The SHM feature was deprecated when HAM8 was introduced. [2] The advantage of SHM files was the ability to display all 4096 colors while eliminating the color blur of HAM compression.


Which is amazing!

marcfrick2112 13 November 2008 16:03

Not aware of SHAM ??? For shame! I ran into a few pics back in the day when I called Amiga BBS's (Mostly 'Naughty' pictures :laughing) Man, I remember when there were like, 6 Amiga-supporting BBS in my city

AmigaFriend 13 November 2008 19:01


Originally Posted by marcfrick2112 (Post 476554)
Not aware of SHAM ??? For shame! I ran into a few pics back in the day when I called Amiga BBS's (Mostly 'Naughty' pictures :laughing) Man, I remember when there were like, 6 Amiga-supporting BBS in my city

Yup, my fault!:D

Si-Pie 13 November 2008 19:38

Ok anyone have any idea of how much cpu usage HAM would use? Would it be heavy or light?

LocalH 13 November 2008 21:06

Depends on how you're using it. For displaying static images, it takes no extra CPU. However, when using programs like DPaint in HAM, it takes extra CPU to calculate the "transition" pixels when the image is updated.

Si-Pie 13 November 2008 21:15

OK thanks. Think I was thinking of memory cycles as your right LocalH with static images it uses no extra cpu cycles.
I found the Amiga Hardware Reference Manual and it said this "If the display contains four or fewer low-resolution bit-planes, the
68000 can be granted altemate memory cycles (if it is ready to ask for
the cycle and is the highest priority item at the time).
However, if there are more than four bit-planes, bit-plane DMA will begin
to steal cycles from the 68000 during the display.

During the display time for a six-bit-plane display (low resolution, 320
pixels wide), 160 time slots will be taken by bit-plane DMA for each
horizontal line. As you can see from Figure 6-11, bit-plane DMA steals 50
percent of the open slots that the processor might have used if there
were only four bit-planes displayed."

Think that answered my question. Its memory cycles it uses not CPU cycles.

AmigaFriend 13 November 2008 22:18

If not, the rare HAM games that were made, wouldn't be made at all!

Si-Pie 13 November 2008 23:50

Very true xpect. Its settles the debate I was having with someone anyway as they were claiming HAM mode was heavy on the CPU usage and so much that the CPU couldnt do anything while HAM was being used but I thought this was wrong. Im glad I found the Amiga hardware reference guide to. Thanks you guys for your help too!

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