microcode store (was Re: Looking to get into a MiniComputer...)

Even though IBM tout compatibility even System/360/370/XA/System/z has "lost" instructions.... The original 360 had Read Direct and Write Direct which as fa as I can tell accessed a socket somewhat akin to a bi-directional printer port.. .. but I have never seen any pictures.... Then the Vector Facility instructions have been re-used. Of course these needed a vector facility hardware thingy, which is no longer made so I suppose that's fair enough.... Dave G4UGM -----Original Message----- From: cctalk-bounces at classiccmp.org [mailto:cctalk-bounces at classiccmp.org] On Behalf Of Peter Corlett Sent: 30 July 2014 17:46 To: General Discussion: On-Topic and Off-Topic Posts Cc: General Discussion: Subject: Re: microcode store (was Re: Looking to get into a MiniComputer...) On Tue, Jul 29, 2014 at 09:13:42AM -0700, Chuck Guzis wrote: [...] Reasonably interesting CPUs effectively have two instruction sets: user instructions in which applications are implemented, and system/supervisor instructions for use by the operating system. The latter are more malleable than the former, since the operating system is expected to be upgraded to take the different functionality into account. As an example of changes to the OS functionality, the 68030's MMU offered a wide range of page table sizes, but in the 68040 this was reduced to just two, with corresponding changes to the supervisor instruction set. When Freescale evolved the 680x0 line with their "Coldfire" series of CPUs, they also removed a whole swathe of instructions and addressing modes such as DBcc and BCD instructions. At one point in ARM's evolution, they added a new operating mode called Jazelle which could execute the simpler JVM instructions and would trap for hard-to-implement instructions. Contemporary ARM chips have stub support which trap on *all* instructions. When x86 was extended to 64 bits, segmentation support was gutted (yay!) and remain as just a pair of extra base registers, typically used to locate thread-local storage.