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Mesa.mu 2-Sep-78 17:21:54 Page 1 t , , 0 _________________________________________________________________ M E SAM I C ROC 0 D E Version 34 , 0 ______ - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - ____________ , t ; Mesa.Mu - Instruction fetch and general subroutines ; Last modified by Levin - August 1, 1978 11:27 AM ;Completely rewritten by Roy Levin, Sept-Oct. 1977 ;Modified by Johnsson; July 25, 1977 10:20 AM ;First version assembl~d 5 June 1976. ;Developed from Lampson's MESA.U of 21 March 1976. ;----------------------------------------------------------------- ; GLOBAL CONVENTIONS AND ASSUMPTIONS ;----------------------------------------------------------------- 1) Stack representation: stkp=O a) stack is empty sktp a 10 =) stack is full The validity checking that determines if the stack pointer is within this range is somewhat perfunctory. The approach taken is to include specific checks only where there absence would not lead to some catastrophic error. Hence, the stack is not checked for underflow, since allowing it to become negative will cause a disaster on the next stack dispatch. 2) Notation: Instruction labels correspond to opcodes in the obvious way. Suffixes of A and B (capitalized) refer to alignment in memory. 'A' is intended to suggest the right-hand byte of a memory word; 'B' is intended to suggest the left-hand byte. Labels terminating in a lower-case letter generally name local branch points within a particular group of opcodes. (Exception: subroutine names.) Labels terminating in 'x' generally exist only to satisfy alignment requirements imposed by various dispatches (most commonly IR~ and BIA in instruction fetch). 3) Tasking: Every effort has been made to ensure that a 'TASK' appears approximately every 12 instructions. Occasionally, this has not been possible, but (it is hoped that) violations occur only in infrequently executed code segments. 4) New symbols: In a few cases, the definitions of the standard Alto package (ALTOCONSTS23.MU) have not been quite suitable to the needs of this