Amiga HAM

Si-Pie · 10 November 2008, 19:36

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:

http://en.wikipedia.org/wiki/Hold_And_Modify

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 (

) 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

) Man, I remember when there were like, 6 Amiga-supporting BBS in my city

AmigaFriend · 13 November 2008, 19:01

Quote:

Originally Posted by marcfrick2112

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

) Man, I remember when there were like, 6 Amiga-supporting BBS in my city

Yup, my fault!

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!

10 November 2008, 19:36	#1
Si-Pie Registered User Join Date: Jun 2008 Location: Newcastle Under Lyme/England Age: 46 Posts: 93	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?

10 November 2008, 20:00	#4
Charlie . . Mouse . . Join Date: Jun 2005 Location: Nowhere Age: 55 Posts: 1,792	HAM mode is indeed a deliberate part of the Amiga Chipset design. Interesting fact: If memory serves Jay Miner () 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...

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10 November 2008, 19:45	#2
AmigaFriend Registered User Join Date: Sep 2008 Location: CLI Posts: 1,462	As always on these days: http://en.wikipedia.org/wiki/Hold_And_Modify If anyone can provide more info...

10 November 2008, 19:54	#3
Si-Pie Registered User Join Date: Jun 2008 Location: Newcastle Under Lyme/England Age: 46 Posts: 93	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 ....

10 November 2008, 20:03	#5
Si-Pie Registered User Join Date: Jun 2008 Location: Newcastle Under Lyme/England Age: 46 Posts: 93	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.

10 November 2008, 20:56	#6
AmigaFriend Registered User Join Date: Sep 2008 Location: CLI Posts: 1,462	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!

13 November 2008, 16:03	#7
marcfrick2112 Amiga Fanatic Join Date: Apr 2008 Location: Phoenix, Arizona, USA Age: 53 Posts: 179	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 ) Man, I remember when there were like, 6 Amiga-supporting BBS in my city

13 November 2008, 19:38	#9
Si-Pie Registered User Join Date: Jun 2008 Location: Newcastle Under Lyme/England Age: 46 Posts: 93	Ok anyone have any idea of how much cpu usage HAM would use? Would it be heavy or light?

13 November 2008, 21:06	#10
LocalH Amiga user since 1990 Join Date: Aug 2004 Location: Kingsport, TN / USA Age: 44 Posts: 295	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.

13 November 2008, 21:15	#11
Si-Pie Registered User Join Date: Jun 2008 Location: Newcastle Under Lyme/England Age: 46 Posts: 93	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.

13 November 2008, 22:18	#12
AmigaFriend Registered User Join Date: Sep 2008 Location: CLI Posts: 1,462	If not, the rare HAM games that were made, wouldn't be made at all!

13 November 2008, 23:50	#13
Si-Pie Registered User Join Date: Jun 2008 Location: Newcastle Under Lyme/England Age: 46 Posts: 93	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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