From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <004e01c10ddd$691100a0$3cf7c6d4@SOMA> From: "Boyd Roberts" To: <9fans@cse.psu.edu> References: <20010713091130.B8A0D199C0@mail.cse.psu.edu>, <20010713182020.C22003@cackle.proxima.alt.za> <3B4F350B.EDDD098C@null.net> Subject: Re: [9fans] how people learn things (was architectures) MIME-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit Date: Mon, 16 Jul 2001 11:55:14 +0200 Topicbox-Message-UUID: cec6bfba-eac9-11e9-9e20-41e7f4b1d025 From: "Douglas A. Gwyn" > The PDP-11 had a segmented architecture in the sense that there > were 8 8KB "pages" in the (D or I) 16-bit address space, each > mapped by an 18-bit offset (constrained modulo 64 as I recall) by > a register in the Memory Mapping Unit. 18 bits, that sounds right [128kw]. later there was separate I & D; two 64 bit address spaces, one for instructions and the other for data. > Later PDP-11s added a second level of memory management to implement > 22 bits of real address space. Addresses in opcodes were (at most) > 16 bits. In "kernel mode" (the equivalent of a PC's "real mode"), > such as under the RT-11 SJ monitor, a user program could load the > mapping registers directly whenever it wanted (although doing it stupidly > would just cause a crash). iirc the 11/70 had kernel/executive/supervisor/user. > Berkeley did something similar at the same time, but my "thunks" > were smaller and faster.) yes, this was 2.10 bsd? i remember porting the required bits to an 11/23 kernel so it could run vi. > MOV R0,(R0)+ was implemented incorrectly on the PDP-11/20, and > the spec for the ISA had to be changed to reflect that. Later > models got other things wrong; programs could in fact tell models > apart by their semantics for execution of particular code sequences. yep, i remember some of the generic kernel boot code (written by chris maltby?) that exploited this to work out what sort of an 11 it was. > PDP-11s were designed without thought being given to test-and-set. iirc, this was added to the vax, but i don't have a 'blue' book. > I will say that we took a great step backward when > chip manufacturers took over architectural design. It seems that > every "generation", people reinvent things without taking into > account lessons learned from the previous cycle. couldn't agree more.