From: ??? (wegaia@micro-web.co.kr)
Date: Fri Dec 14 2001 - 04:10:06 EST
Thanks for the help Park. ----- Original Message ----- From: "Philip Nye" <philip@engarts.com> To: <uclinux-dev@uclinux.org> Sent: Thursday, December 13, 2001 9:43 PM Subject: Re: [uClinux-dev] [Newbie] why use CPUs without MMU? > > From: "¹Ú¿µÈÆ" <wegaia@micro-web.co.kr> > > > > Hi > > i'm studying Samsung 50100 CPU using ARM7TDMI core. > > i wonder why that company use MMU-less CPU... > > it's the matter about cost? or peformance? > > Thanks. > > Both - but mainly cost. > > Cost: > An MMU takes up a significant amount of silicon space and therefore costs > money. Writing code to drive it also takes memory space which costs money. > This is not justified in many embedded systems where the program structure > is simple or if there are not sufficient memory resources to warrant > management. > > In general to benefit from an MMU you need to be running a fairly > sophisticated OS and this will also take more memory and other resources so > the cost is not just the MMU. > > You could compare the prices of ARM7 and other comparable processors without > MMUs (e.g. Coldfire) with chips which include MMUs such as ARM9s (also a few > ARM7s), PowerPC etc. > > Performance: > An MMU needs to intercept memory accesses and redirect them - this takes > time and so degrades raw performance. In many processors the performance > degradation can be largely masked by pipelining and cacheing but this again > adds complexity and so costs money. The performance benefits of an MMU are > in overall system performance rather than raw memory performance. > > > Philip Nye > > > This message resent by the uclinux-dev@uclinux.org list server http://www.uClinux.org/ > This message resent by the uclinux-dev@uclinux.org list server http://www.uClinux.org/
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