xerox Alto II Microcode Aug75 free download

File name Alto_II_Microcode_Aug75.pdf

Inter-Office l\'Temorand urn To Alto Users Date 19 August 1975 From C. Thacker location Palo Alto Subject Alto II Microcode Organization CSL XEROX This memo is a description of the changes which have been made in the standard Alto microcode for the second build. These changes will not affect the operation of the present software (except to make it work better in some cases), but they will make it possible to run Lisp, and make certain const.ructs simpler for those who wish to wri te special purpose microcode. The changes are: 1) A bug which sometimes caused a 1\1:44 disk to get spurious da ta late errors when the processor executed the BLT or BLKS instruction has been fixed. 2) The constant memory has been expanded. 3) Unused opcodes 61022-61037, which if executed would produce bizarre results, now trap. 4) A bug which caused unused parameterless opcodes in the ranges 61013-61017 and 61022-61377 to occasionally trap to the incorrect location has been fixed. This bug will be fixed on all present machines, as well as on the second build. 5) Currently, all unused opcodes which replace the Nova I/O instructions trap to a location determined by bi ts 3-7 of their opcodes. There is a prom (DISP) which maps these instructions into the single microlocation TRAP. The Bytelisp system requires that this be changed; All unused opcodes except 77400b (which is used by Swat) now transfer to microlocation U16, which executes: U16: SWMODE, :TRAP; The result of this is that if your machine has a control RA1\I, these instructions will cause control to enter it at a location which is equal to TRAP in the PROM microcode. If no RAM is .present, things will work normally. 6) A feature has been added to the Ethernet microcode which causes it to reject truncated packets at the microcode level, rather than posting them to the softv.'are. 7) CurrentJy, the IDISP function produces the follo\ving values, which are ORed with NEXT: IR[01],IR[02] Value 00 IR[02],IR[03] o1 4 10 5 1 1 if IR[03]-IR[07]=16b then 6 else 1 To allow passing of up to 16 return address indices to subroutines, the final case above has been changed to: IR[Ol],IR[02] Value 11 if IR[04]-IR[07]= 1Gb then 6 elsei f IR[04]-IR[07]= 6 then 16b elseif IR[04]-IR[07] = 0 then 1 elseif IR[04]-IR[07] = 1 then 0 else IR[04]-IR[07] The constants srO through sr17 have been defined in the standard constant memory so that a subroutine can be called from one of sixteen places thus