by May 5, 2003 0 comments



To meet the high battery backup needs of mobile device users, chip manufacturers have been putting in extra efforts to make low power CPUs. Earlier it was Intel with the Centrino mobile PC platform and now Transmeta, producer of Crusoe processors, with its TM8000 processor, code-named
Astro. 

The Astro also uses the same concept as the Crusoe, which is different from other processors in that it is a mix of hardware and software. Crusoe understands VLIW (Very Long Instruction Word) instructions that let it run multiple instructions in one clock cycle. To run x86 programs, the x86 instructions from the applications are translated to VLIW instructions using code-morphing software. This moves some of the complexity of the processor into software, reducing the number of transistors on the chip. Fewer transistors also means less power consumption and thus less heat generation. 

The Astro utilizes this concept but with 256-bit VLIW instructions, instead of the 128-bit in the Crusoe. This lets it execute eight separate 32-bit x86 IPC (instructions per cycle) compared to the typical four. Since each clock cycle consumes power, by executing multiple instructions in a single cycle, more work is done per cycle, thus saving power. 

The best part of the Astro is that it comes with integrated on-die Northbridge core-logic technology. Generally a processor communicates with the Northbridge, which in turn communicates with the system memory, AGP and Southbridge, which talks to I/O devices. In the Astro, these three bus interfaces are built inside the processor. First is the HyperTransport bus that speeds up I/O tasks. This bus, developed by AMD, is a high-speed link for interconnecting integrated circuits and devices on the motherboard. The second is the DDR 400 memory interface, which improves memory throughput by talking directly to the memory and eliminating the Northbridge. Third is the AGP 4x interface through which the AGP card talks to the processor, bypassing any intermediate circuitry. This improves graphics performance significantly. By putting this functionality within the processor, a separate Northbridge is not required on the motherboard, thus removing some integrated circuits off the motherboard. This way the Astro helps increase the performance, decrease the number of chips and circuits required for a complete system, brings cost and performance benefits to system manufacturers and improves battery backup. 

APM vs ACPI

  APM  ACPI
How
it Work
BIOS detects ‘idle’ state 
for devices and turns off  the power.
The OS controls when
to switch off power to a  device.
Pros Has wider adoption and, 
hence, more support in  OS and BIOS software.  Even the new Windows OS
comes with APM as well as 
ACPI support. 
More control to the OS,
makes power management  intelligent and more flexible  than it was earlier. 
Cons It can cause a system to
suspend operations even  while actually being used,  by detecting an idle state, 
for example, in the middle   of a presentation or during 
data transfer over the Internet.
ACPI has yet to gain more 
widespread  adoption to become more robust as a  specification. 

The Astro also comes with 1 MB of L2 cache as against the 512 KB of Crusoe. More cache means more data can be found in the cache thus not accessing the main memory and saving valuable power. 

The processor would be available in the latter half of this year and will run at speeds more than 1 GHz. It is being fabricated using 0.13 micron semiconductor technology. It’s said that the Astro’s power requirement is so low that it does not require a noisy fan to run, even for processors meant for desktop systems. When you consider the big P4 processor fans, this is very interesting.

Anoop Mangla

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