Why XIP doesn't get so much use with Linux...
Posted Sep 5, 2008 0:10 UTC (Fri) by HalfMoon
Parent article: AXFS: a compressed, execute-in-place filesystem
XIP is less interesting now than it was a few years back. The thing is that it basically applies only to NOR flash ... but it's NAND which is getting the huge price shrinkage. I suppose folk developing ROM images could also use XIP, but that just seems to emphasize the point that the audience here doesn't seem to be one with long term growth.
A technical reason why XIP isn't as interesting lately include that the access times hurt ... it's pretty much always a lot faster to run from RAM. So you don't want XIP for frequently-used code.
There are places where XIP matters a lot, though many of them don't run Linux. Some won't even run uClinux. Any place you see microcontrollers in use, it's likely you're talking about System-On-Chip processors with integrated flash used for program memory. Things like AVR8 (and some AVR32), MSP430, PIC (sigh), ARM7TDMI, C2000 ... where 256KBytes of flash is a rather large part, as is 32KB of RAM, and a 60 MHz clock rate is pretty fast.
XIP with uClinux can be a way to keep system costs low though. If you're using only a 4 MB flash, and 16 MB of RAM, you're probably pretty close to the performance edge. Getting a few extra MBytes of working memory by using XIP can help shave dollars from your BOM.
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