Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!think.com!spool.mu.edu!uunet!mcsun!ukc!tcdcs!maths.tcd.ie!ecarroll From: ecarroll@maths.tcd.ie (Eddy Carroll) Newsgroups: comp.sys.amiga.advocacy Subject: Re: Blitter vs. 040 (was: Computer Architecture question Message-ID: <1991May15.150357.15358@maths.tcd.ie> Date: 15 May 91 15:03:57 GMT Article-I.D.: maths.1991May15.150357.15358 References: <3310.tnews@templar.actrix.gen.nz> <00674001103@elgamy.RAIDERNET.COM> Organization: Dept. of Maths, Trinity College, Dublin, Ireland. Lines: 44 In article <00674001103@elgamy.RAIDERNET.COM> elg@elgamy.RAIDERNET.COM (Eric Lee Green) writes: > but the A3000 still has twice the bandwidth into video >memory, compared to the typical '386-based PC. Probably closer to five times in the low-resolution modes and ten times for (four-colour) hires, since standard VGA only gives either 1 or 3 out of every 5 cycles to the CPU (the rest are needed for screen refresh), and the A3000 has a 32 bit path to chip ram as opposed to VGA's 16 bit (or even 8 bit!) >Note that there exists software patches for the A3000 to patch the OS's >BltBitMap etc. calls to use the CPU instead of the blitter... but most >people still don't do it, because other processes slow down drastically >when so much CPU is being used to shove bytes around on the screen. (Even >though it DOES speed up the screen scrolling and such). n As author of one such program (CpuBlit), I'd like to make a small correction. CpuBlit has a variety of options that let you configure it to only use the CPU for blitting if there are no other processes waiting to run. This way, you get the increased speed when available without paying the penalty in system performance. (Check comp.sources.amiga for the latest version.) Even using the 68030, you don't get much better than a doubling in speed, and this can be attributed to the 32-bit access the 68030 has to CHIP ram. Note that this is also only for the simplest possible blit: copying memory without changing the bit alignment. CpuBlit doesn't even attempt to handle blits that are more complicated than this, since it works out much slower than letting the blitter do the work. If you get the chance (I don't mean Eric specifically) have a look at the source code for a Windows 3 display driver, and look at how much work is needed to do a general three-operand blit in software; it's HUGE (Yes, I know the Amiga's blitter doesn't handle all the cases that Windows does). As someone pointed out already, the limiting factor is the speed of the custom chip bus, and the blitter already utilises that pretty fully (albeit at 16 bits). Other than the 32 bit vs 16 bit speedup, there's not much scope for improvement (except for a new, faster custom chipset of course :-) Eddy -- Eddy Carroll ----* Genuine MUD Wizard | "You haven't lived until ADSPnet: cbmuk!cbmuka!quartz!ecarroll | you've died in MUD!" Internet: ecarroll@maths.tcd.ie | -- Richard Bartle