Most history routines defined a local sim_eval of the proper length, but the
erroneous ones were fixed length machines that defined an integer variable
instead of an integer array of length 1. The VAX used the global sim_eval.
The changes follow the VAX practice.
The major change is the implementation of deferred IO - a more
accurate implementation of the 1620's "stop in its tracks" IO model.
When a device uses deferred IO, instruction execution is suspended
until the IO completes successfully. Operator interruptions, errors,
and so on do not return to instruction execution; this only occurs if
the IO completes successfully or the command SET CPU RELEASE is given
(equivalent of pressing the RELEASE button). Otherwise, the current IO
operation continues to execute.
Only the console typewriter and paper tape reader/punch currently
implement deferred IO; there are operational issues with those devices
that require more accurate modeling. The card reader/punch, line
printer, and disk still execute IO "instantaneously". It's not all
that hard to convert an instantaneous device to deferred operation,
but there's no point in doing so (and possibly introducing new bugs)
unless there's an actual operational issue. The 1620 doesn't have
overlapped IO, so programs can't tell the difference, by and large.
A number of other issues have been addressed as well, including the
bizarre "treat RM as 0 in the Q field" required by MI-015; the
treatment of non-existent indicators as always off; and various other
tweaks.
I've run CU01 (again), which at least gives typewriter and paper-tape
IO a basic workout; and it works. I leave more detailed testing to
people who know the machine better than I do.
The documentation has been updated to include Tom's detailed breakdown
of IO handling for all IO operations on the typewriter, paper-tape
reader/punch, card reader/punch, and line printer.
It turns out that the two reversed opcodes Maurice identified were not the
only problems in opcode 12 (shifts). All of the INS/EXT pairs at function
codes .57 and above were reversed. In addition, the mnemonics in the
opcode table in alpha_sys.c are wrong as well as reversed.
- Changed to commit PC on certain stops
- Added SET CPU RELEASE command
- Undefined indicators don't throw an error (Dave Wise)
- Added Model I mode to allow record marks in adds (Dave Wise)
- Allowed undocumented indicator 8 (Dave Wise)
- Added option for Model I diagnostic mode (Dave Wise)
From page 6-6 of DEC STD 032 (the VAX architecture spec):
"Execution of MTPR src, #PR$_ASTLVL with src<31:0> GEQU 5 results in
UNDEFINED behavior. The preferred implementation is to cause a reserved
operand fault." MicroVAX II, CVAX, and Rigel all conform to the preferred
behavior, as does the current simulator, which was written from the CVAX
microcode. NVAX masks to 3b and does not take an exception on a value
GEQU 5.
The 1982 Architecture Handbook describes ASTLVL as a 3b register, with
src<31:3> ignored/read as zero, and exceptions taken on values GEQU 5.
The780 microcode masks the input value to 3b before doing the GEQU 5 test.
The ASTLVL test needs to be model specific.
I suspect the behavior became undefined when MicroVAX II simplified the
original test to save a microword. I do not see how the code fragment Matt
references could work on a MicroVAX II, which was supported under 4.5.
Perhaps the device Matt mentions couldn't exist on a MicroVAX II?
For those who wants the gory details... uVAX, CVAX, and Rigel do an
unsigned compare on the unmasked src and the constant 5. Carry out
means reserved operand. Overflow is ignored. So an input of 0x80000002 -
0x00000005 (done in the data path as 0x80000002 + 0xFFFFFFFB) generates overflow (ignored) and carry out.
Maurice Marks, at VMS Software Inc, is using the Alpha instruction emulator
from SimH as a backup to an Alpha binary translator! He found a bug in the
instructions definitions: function fields, both integer and floating, are 7b not
6b.
the opcode table first word consists of bits:
<7> = FPD is legal for this opcode.
<4:6> = number of specifiers for unimplemented opcodes (VAX subsets)
<3> = unused
<0:2> = number of specifiers
The mask used to be 0x70. The convention is that x_M_y is a mask value
<right-justified>, for a macro like:
#define get_foo(x) (((x) >> x_V_foo) & x_M_foo)
For a subset VAX (like the 3900), the unimplemented opcodes are those
that are recognized on a full VAX but simply trap as reserved instructions
(e.g., the H-floating instructions). This really only affected symbolic
printing and decoding of these instructions.
The code wasn't handling interrupts correctly when TCCM was written. In
particular, if the GO bit clears DONE, the code looked for the "SET IE" case
too soon and didn't clear the interrupt that was incorrectly generated.
The new code factors "GO" into the calculation.
