Follow Slashdot blog updates by subscribing to our blog RSS feed


Forgot your password?

Comment: Great news (Score 1) 63

by AaronW (#49565947) Attached to: Imagination To Release Open MIPS Design To Academia

This is great news. When I was in college the microprocessor design class used a variant of MIPS though this started the quarter after I took the class. In my class we had to wire-wrap a 16-bit MIPS-like CPU using discrete chips and a couple programmable ones. MIPS is relatively easy to implement for educational purposes due to the simple instruction encoding and clean architecture. MIPS, unlike some other processors like ARM, also allows you to add your own instructions using coprocessor 2 which can be a great way to differentiate a processor and enhance it for different tasks. MIPS is a much simpler design compared to ARM or even PowerPC. MIPS is still widely used, especially in networking devices.

For example, my current employer has added a lot of instructions useful for encryption and hashing as well as some useful atomic instructions.

The move from 32-bits to 64-bits is actually quite clean on MIPS which did not require any major changes to the instruction set other than adding 64-bit instructions and sign-extending the 32-bit instructions. There are a few warts on it, such as the fact that the instruction following a branch instruction is always executed (SPARC is the same way). This is no longer all that useful with modern superscalar architectures and the branch delay slot can't always be filled with something useful. Things are also a bit cramped for 32-bits since only the lower 2GB of memory is available for user-space. Kernel space (KSEG0) is from 0x80000000-0x9fffffff and an uncached copy is at 0xa0000000-0xbfffffff. 0xC0000000 - 0xDFFFFFFF can be mapped using the translation look-aside buffer, making another 512MB available to the kernel. Both of these address ranges map directly to the lower 512MB of RAM which somewhat limits things. In 64-bit mode this isn't a problem though since all addresses are sign-extended. Another nice feature in kernel mode is that all physical addresses can be directly accessed, without requiring any special mapping other than setting bit 63 to 1.

Most MIPS processors do not use a hardware page table walker, instead relying on a software configured translation look-aside buffer. When there's a page miss, a quick interrupt occurs to replace an entry in that table with some hardware assist. This goes back to the original philosophy of keeping the hardware design simple. Due to the way it works, it allows total freedom with how page tables are represented in the operating system though there can be a slightly bigger overhead compared to hardware page tables.

The instruction set is quite clean and the instruction encodings are quite simple with only a few classes of instructions, unlike ARM64. Instruction decoding can be handled with only a few look-up tables. MIPS assembly language is far simpler and straight forward than, say X86 and it's quite mature, though not all processors implement all instructions. Many embedded MIPS processors lack floating point and the multimedia extensions, and many are 32-bit only. This helps cut cost and power when making chips for devices that don't need these features. MIPS also can scale up nicely. For example, the chips I work with currently scale up to 48 cores per chip and with two chips running in tandem Linux runs on 96 cores, all with a coherent cache. The newer ABIs are nice in that the only real difference between N32 and N64 is that pointers are 64-bits instead of 32-bits, just about everything else is the same so you get all of the features of 64-bit registers but keep the compactness of 32-bit pointers. This has been present for many years and is a fairly recent addition to X86 and ARM.

Comment: Re:Many small solutions through a day (Score 1) 174

by AaronW (#49547889) Attached to: Apple Watch Launches

I wear my watch all the time, including in the shower. I like it because it does one thing well, tell me the time and date. I never have to set it except for daylight savings (it synchronizes itself every night) and I don't have to wind it up or charge it (it charges itself via solar). It's also waterproof to 100M and pretty tough (I tend to be hard on watches). If it breaks I'm also not out $600 and it won't go obsolete in a year.

Comment: Re:Landing vs splashdown (Score 1) 342

My father was is engineer who worked on some aerospace and defense stuff while I was growing up. He worked on a data recorder that went up on Skylab. He offered to fly up and fix it if anything went wrong. My mother has always been big on astronomy. I've been Elon Musk's various ventures for quite a while (and ended up buying a Tesla). I've always been impressed with his forward thinking and the smart people he surrounds himself with. My day job is working on the U-Boot bootloader and a little Linux kernel work. Maybe someday it'll even be pushed upstream, but right now it's always not enough time and it's a LOT of code, significantly more than all of the ARM SOCs combined.

Comment: Re:Landing vs splashdown (Score 4, Interesting) 342

A couple of months ago I was having a discussion with a fellow from Space X who designs the hydraulic systems and we spoke about a number of issues. This was right after the failed landing due to it running out of hydraulic fluid. I asked about how reusable the engines are and he said that they run test burns lasting hours. The launch is only a few minutes. According to what he said, it should just be a simple matter of refueling and adding more hydraulic fluid and probably some other simple things without having to do a major overhaul. The engines are very reliable.

I asked about why they don't reuse the hydraulic fluid and he said that it was cheaper and lighter to not reuse it. He also said that they knew it could run out and that the next version would have more.

Comment: Re:Not Brick (Score 2) 179

by AaronW (#49451519) Attached to: Google Lollipop Bricking Nexus 5 and Nexus 7 Devices

I had this problem and after trying several things I finally found a solution: Buy an Anker USB cable. I tried just about everything else with my tablet. I have a USB power meter. The Nexus 7 2012 seems to be extremely sensitive to the resistance in the USB cable. The Anker cable is much fatter than most cables I've tried. Before I switched to this cable I've had my tablet go dead while plugged into the charger.

Here's what else I tried:
I replaced the USB connector on my tablet (fairly easy to do). This helped but I still had problems.
I tried numerous chargers including the Anker charger, which helped but didn't solve the problem.
I tried many different USB cables. Some would help briefly but none ever charged quickly, the best one was the stock one that came with the tablet, but even that didn't work too well.

Once I switched to the Anker cable I was able to charge at over 1A for the first time. None of the other cables came close and I tried a lot of cables, including the Amazon Basics cables (which are otherwise nice cables).

Comment: Re:Missing the point. (Score 1) 330

by AaronW (#49407727) Attached to: Inexpensive Electric Cars May Arrive Sooner Than You Think

Hydrogen is not cleaner and generates more greenhouse gases than a decent hybrid or diesel vehicle. The only way to make hydrogen in an affordable manner is to crack natural gas. You use 20% of your energy capacity of the hydrogen just compressing it. And the hydrogen must either be generated on-site (around 70% efficient) or transported in tanker trucks which aren't very efficient for the energy density. Fuel cells also suck. They're maybe 50% efficient at best and quickly start to degrade. By 70K miles the fuel cell output has dropped to around 70% of its rated capacity and have dropped considerably in their efficiency.

The Tesla battery typically contains over 90% of its original capacity at 100,000 miles.

Fuel cell cars right now are very heavily subsidized and don't make a lot of sense. They're certainly not very green when you take into account the well to wheel efficincy. A hybrid vehicle or a diesel vehicle is more efficient than a hydrogen fuel cell vehicle.

Comment: Re:Reeeal modern, like before Amazon (Score 1) 197

That's what I ran into years ago when I ordered my Prius. I could get any car I wanted as long as it was white or possibly black. I wanted neither and a certain set of options. I had to wait 6 months to get the car I wanted, and it was my 3rd choice for color. They kept offering me white cars which I didn't want.

When I ordered my Tesla I chose exactly what color and options I wanted. I still had to wait 6 months but I got exactly what I ordered. The service I've gotten from Tesla is far better than the service I have gotten at any dealership by far.

The dealers like to make the claim that they protect the consumer. This is total BS. My father bought a Fisker Karma (I tried to talk him out of it) and once they went bankrupt, so did his service and warranty support.

Comment: Re:First attack 2nd amend, and then 1st amend (Score 1) 538

To understand her position you need to understand where she's coming from. She's probably one of the few politicians to have seen gun violence first hand. She was there when the SF Mayor George Moscone and Supervisor Harvey Milk were assassinated. She grabbed Harvey Milk's wrist to check for a pulse and her finger entered one of Milk's bullet wounds and was badly shaken by the event.

See the Moscone Milk Assassinations.

Comment: Re:Woop Di Do Da! (Score 5, Insightful) 265

The sad part is that states like Florida are making it harder to install solar. On top of that, Florida is fighting energy efficiency. Other states are adding fees to solar users at the behest of the utility companies.

I live in California and am getting solar installed later this week though not nearly as big of a system as I'd like due to limitations of my roof. PG&E has some of the most expensive electricity in the country because of our state's corrupt public utilities commission. Average rates are around $0.194/kwh (compared to Santa Clara $0.113/kwh). PG&E has been quietly lowering the thresholds to push people into higher tiers of power as they make their homes more energy efficient. On average I'm paying well over $0.19/kwh so solar makes perfect sense.

If God had a beard, he'd be a UNIX programmer.