:TITLE[Fault];*Fault handler. 1 June 1982 by Ed Fiala
* Edited October 2, 1982 5:19 PM van Melle
* Lisp conditional at MemFault fixed June 22, 1982 4:57 PM van Melle

%Problems:
1) PFetch4; NextInst will normally fault abort at MesaRefillLoc and will be
treated as a NextInst refill fault instead of ordinary reference; can
improve things by ensuring task 0 from PipeReg2 (almost in SALUF?).
2) Try to do ReadPipe only on MC12Err path, if that path not made longer.
3) Do something useful in the mi at MC12Err+2.
4) MP report of storage failures would be desirable.
5) If branch condition on first ReadPipe mi could be eliminated (2 above),
then could pick off MC1 crash in that mi by testing for FFault odd; then
test for CTask=0 in the bare Disp at MC12Err+2 and branch on ALU#0 in
the dispatch table to an mi that does RTMP←. This would save 2 cycles
on the slowest paths.

DELAY ON FAULTS until tasking MUST BE MINIMIZED to avoid data lates on io
devices. Although present io devices eventually recover from data lates,
keyboard input is lost (primarily on Star keyboards--CSL keyboards are
rarely affected) and display jitter occurs; there are other potential
problems.

In computing time for an MC1 fault, allow ~6 cycles after the PFetch/PStore;
the mi after that is aborted until the fault starts; the abort lasts
~10 cycles, unaffected by suspended cycles while transport for previous
references takes place. Then the code here beginning at "FaultOccurred"
is executed. The assumption is made that only the emulator task can
legitimately have references that fault, but the fault may not occur until
another task is running.

In the event the emulator was running at the time of a fault, it is
restarted at a fault address and tasks after a delay of 51 to 69 cycles.
In this case, io task timing is worsened by ~61 to ~79 cycles minus the time
to the next normal emulator tasking point. The next two emulator tasks
occur after 11 and 4 cycles, respectively, which somewhat mitigates the long
time to task.

If an io task was running, however, the emulator TPC is changed to a fault
address and the state of the io task is restored after ~83 to ~100 cycles;
in this case, time in the fault handler is totally additive to worst
case timing in the absence of a fault.

An even longer time in the fault handler (up to ~110 cycles) occurs when
an io task was running at the onset of the fault and doing a "Return" to
the emulator. However, this only happens when no other io task was
requesting service at the onset of the fault, so it is not as bad as the
additive ~100 cycles.

Code here replaces that in Kernel or Initial performing the same function.
This code determines what to do based on the type of error, and bits in
FFault, which are:
0: MC2 errors Return if 1, crash if 0. The return option is used by
Initialize.Mc during storage initialization when WithMidas=1 (i.e.,
on debugging systems, not on release systems). This bit is reserved
for Initial’s MemInit storage diagnostic on release systems.
2: H4 parity errors ignored if 0, crash if 1
(3 MB Ethernet generates them routinely)
3: Midas is present (1), so "crash" means breakpoint, else put a code in MP
and GoTo[.] until booted.
15: MC1/StackOvf errors handled by notifying PFEntry in emulator (1), or by
crashing (0); the Alto emulator crashes, the Mesa emulator uses a
notify for these errors [On 1 June 1982, this bit wound up in the wrong
state after LoadRAM.].
%

SetTask[17];

LoadPageExternal[0], GoToExternal[377], At[0];*Buffer refill trap

FaultOccurred:
T ← APCTask&APC, At[1];*Must save APC first
RXAPC ← T;
T ← (CTask&NCIA) xnor (170000C);
RXCTask ← T;
*It is necessary to ensure that StkP .ne. 17b here because, even though
*ResetErrors clears stack overflow, StkP .eq. 17b instantly regenerates it
*causing an endless loop back to location 1. So set StkP to a value that
*will be used later.
T ← RCy[Page&Par&Boot,4];
*LoadPage is necessary because, after ResetErrors, the Page register will no
*longer be disabled by the error condition.
RXPPB ← T, LoadPage[FaultPage], GoTo[NotStkFault,H2Bit8’];
T ← (SStkP&NStkP) xor (377C);

OnPage[FaultPage];

RXSTK ← IP[RXCTask]C;
StkP ← RXSTK, RXSTK ← T, NoRegILockOK;
*aluresult, saluf (both read complemented)
NotStkFault:
T ← (ALUResult&NSALUF) xnor (0C);
RXALU ← T, ResetErrors;
*Test first for time critical MC1/2 error without any accompanying StkOvf,
*CSPE, or RMPE. Can’t change ALU branch conditions until after ResetErrors.
*The four bits tested here are the Parity register (Stack overflow, CS parity
*error, RM parity error, and Memory error. Memory errors are time-critical,
*so test for them first.
LU ← (LdF[RXPPB,10,4]) - 1;
*Get pipe A (Timing for this mi = 8 cycles on MC12Err)
ReadPipe[PipeReg], FreezeResult, GoTo[MC12Err,ALU=0];
LU ← (RXPPB) and (140C), Skip[ALU>=0];
*Assume Midas breakpoint--i.e., SetFault function executed.
T ← BrkPCrash, GoTo[CTaskCrash];
T ← RMCSCrash, GoTo[BadHWErr1,ALU#0];*Jump if CS or RM PE
*Have to do ResetMemErrs in case stack overflow occurred in conjunction with
*an MC1/MC2 error.
:UNLESS[LispMode]; *********************************
FFault, ResetMemErrs, GoTo[StackTrap,R Odd];*StkOvf only
:ENDIF; ********************************************
T ← StkCrash, GoTo[Crash];
*CS or RM parity error, possibly in combination with other errors.
BadHWErr1:
T ← (LdF[RXPPB,10,4]) + T, GoTo[Crash];

CTaskCrash:
T ← (LdF[RXCTask,0,4]) + T, GoTo[Crash];

*The computation here is CrashCode+PipeTask = CrashCode+(PipeTask’ xor 17b)
*= CrashCode+(17b-PipeTask’) = (CrashCode+17b)-PipeTask’. The value in T
*when we get here is CrashCode+17b.
PipeTaskCrash:
T ← (LdF[PipeReg2,10,4]) - T;
T ← (Zero) - T, GoTo[Crash];

%If Midas is not present, display the maintenance panel code in T and do
a GoTo[.] until booted. Otherwise, do not modify the MP and transfer control
to the Midas Kernel. Except on simple breakpoints, save the CrashCode and
memory error information in RM 100-107 (MP crash code must be .ls 400b to
see it from Midas).
%
:IF[WithMidas]; *********************************
***NOTE: RM 352-353 will be smashed by PNIP.
Crash:LU ← LdF[FFault,3,1], Call[Crash1];
GoTo[.];*Wait for boot...
Crash1:PipeReg5 ← (LSh[PipeReg5,10]) or T, GoTo[PNIP,ALU=0];
*Locate on page 17 with the Midas Kernel so that this code can be easily
*overwritten when Midas isn’t present. Reformat saved Pipe into RM 100-107
*for Midas. Note that pipe info (other than crash code and Task number) is
*only interesting if CrashCode is 200d to 215d (an MC2 error).
LoadPage[MidasPage];
LU ← LdF[RXPPB,10,1];
OnPage[MidasPage];
RXPPB ← LCy[RXPPB,4], GoTo[.+3,ALU#0];*Unrotate RXPPB for Midas
T ← (SStkP&NStkP) xor (377C);
RXSTK ← T;
LU ← LdF[RXPPB,4,4];
RTMP ← Or[And[IP[MapEntry],360],And[Sub[IP[MapEntry],1],17]]C, GoTo[Crash2,ALU#0];
:IF[IFE[MidasPage,17,0,1]]; *********************
LoadPageExternal[17];
:ENDIF; *****************************************
**Unfixup RXCTask for Midas Kernel.
RXCTask ← (RXCTask) xnor (170000C), GoToExternal[MidasBreakLoc];
Crash2:StkP ← RTMP;
T ← LdF[PipeReg,11,7];*Map row address’
PipeReg1 ← (LSh[PipeReg1,7]) xnor T;*Map row’’ u Map column’’
T ← LdF[PipeReg1,2,16], Call[FltPsh];*MapEntry←page no.
T ← (LdF[PipeReg2,10,4]) xor T, Call[FltPsh]; *TaskNumber
T ← (LdF[PipeReg2,14,4]) xor T, Call[FltPsh]; *RefType
T ← RHMask[PipeReg5], Call[FltPsh];*CrashCode
*Card no. (0..7) into CardNumber; offset by 5 to get actual board
*number in card cage. NOTE: 14b in Card implies that this part of
*the pipe is not filled by the reference. (X xor 7) + 5 = 14b - X.
T ← 14C;
T ← (LdF[PipeReg5,4,3]) - T;
T ← (Zero) - T, Call[FltPsh];*CardNumber
*Map Flags (LogSE, WP, Dirty, Ref) into MapFlags
T ← (LdF[PipeReg5,0,4]) xor T, Call[FltPsh];
T ← LdF[PipeReg4,12,6];*Main column address (6 bits)
PipeReg3 ← (LSh[PipeReg3,6]) or T;*x,x,Blk.1’,,main row addr
PipeReg5 ← LdF[PipeReg5,7,1];
T ← LdF[PipeReg3,2,16];
PipeReg5 ← (LSh[PipeReg5,16]) or T;*And Blk.0...
*This is the (15-bit) quadword number within a 128k card.
*Bits 1 and 2 give the block number.
T ← (PipeReg5) xnor (100000C), Call[FltPsh];*QuadAddr
T ← RSh[PipeReg,10], Call[FltPsh];*Interesting syndrome into 107
:IF[IFE[MidasPage,17,0,1]]; *********************
LoadPageExternal[17];
:ENDIF; *****************************************
**Unfixup RXCTask for Kernel.
RXCTask ← (RXCTask) xnor (170000C), GoToExternal[MidasFaultLoc];

FltPsh:UseCTask, Stack&+1 ← T;
T ← 17C, Return;
:ELSE; ******************************************
Crash:Call[PNIP];
GoTo[.];
:ENDIF; *****************************************

OnPage[FaultPage];

%
There are a number of bugs and non-features in the MC1/MC2 error reporting
hardware which account for the peculiar way things are done here; the
comments here are based upon my reading of the hardware drawings and might
be wrong:

1) ResetMemErrs resets the H4PE, MOB, MC1ErA, MC1ErB, MC2ErA, and MC2ErB
flipflops. Also, the next reference reloads the MC1ErA and MC1ErB flipflops.
The next reference using the same pipe will reload the MC2ErA or MC2ErB
flipflop. H4PE can only be reset by ResetMemErrs.

2) H4PE’s do not set MC1ErA or MC1ErB, so it is impossible to report the
reference and task number for these with any certainty. Hence, the MP code
uses +CTask rather than +PipeTask.

3) I am not sure whether or not MOB errors are indicated correctly in MC1ErA
and MC1ErB. Hence, they report +CTask rather than +PipeTask in the MP.

4) MC2ErA and MC2ErB have pipe-specific clocks, so an error indication will
remain true until another reference uses the same pipe. This means that the
reference after one causing an MC2 error won’t disturb its error indication.

5) MC1ErA and MC1ErB have a common clock, and only one of these can be
indicated at a time. The hardware is SUPPOSED to fault before another
reference starts, but if the preceding reference was a PFetch4 with error
correction which didn’t fault, and if the transport for that reference
occurred between the faulting reference and the fault, then one more
instruction will be executed before the fault task executes at location 1.
This instruction could be another reference. If an extraneous reference does
take place, the original MC1ErA/B indication would be replaced by the results
of the extraneous reference, possibly getting the MCNoneCrash MP code.

6) MC2 is never started if MC1 gets a fault, so it is impossible to have
both MC1ErX and MC2ErX indicated at one time.

Timing = 26 cycles from loc 1 to here.
%
MC12Err:
Dispatch[PipeReg,4,2];*Dispatch on H4pe, MapBnd
***This Dispatch is only useful on the MC1/MC2 path.
*Dispatch on MC2ErA’, MC2ErB’, MC1ErA’, and MC1ErB’ bits
Dispatch[PipeReg,0,4], Disp[.+1];
Disp[MC2ErAB], DispTable[4];*None
*23rd? or 24th? bit of memory address = 1 causes MOB.
T ← MOBCrash, FFault, DblGoTo[MOBTrap,CTaskCrash,R Odd];*MOB
%Ignore improbably legit MOB&H4PE on IOStore4 to flush frequently occurring
fake MOB&H4PE by 3MB Ethernet input task. Some 3MB Ethernet controllers cause
H4PE’s on Input’s and IOStore4’s erroneously; XWTask keeps control in xiTask
so that the H4PE will be reported no later than the 1st mi of the next task
to run, so that LoadPage errors won’t happen. We could refine this check by
continuing from H4PE’s only when xiTask is running, but this would require
one additional NOP in xiTask after IOStore4’s or Input’s, and it wouldn’t
work on gateways with more than one Ethernet controller.
%
LU ← LdF[FFault,2,1], Skip;*H4PE
LU ← LdF[FFault,2,1];*H4PE&MOB
*Check ’ignore H4PE’ bit in FFault.
ResetMemErrs, Skip[ALU=0];
T ← H4PECrash, GoTo[CTaskCrash];
T ← LdF[RXCTask,4,4], GoTo[T17RestoreB];

MC2ErAB:
*ResetMemErrs, GoTo[MC22Die], At[MC12,0];*MC2A/B & MC1A/B
*ResetMemErrs, GoTo[MC22Die], At[MC12,1];*MC2A/B & MC1A
*ResetMemErrs, GoTo[MC22Die], At[MC12,2];*MC2A/B & MC1B
ResetMemErrs, GoTo[MC22Die], At[MC12,3];*MC2A/B
*ResetMemErrs, FFault,
*DblGoTo[MC2ErARet,MC2ErA,R<0], At[MC12,4];*MC2A & MC1A/B
*ResetMemErrs, FFault,
*DblGoTo[MC2ErARet,MC2ErA,R<0], At[MC12,5];*MC2A & MC1A
ResetMemErrs, FFault,
DblGoTo[MC2ErARet,MC2ErA,R<0], At[MC12,6];*MC2A & MC1B
ResetMemErrs, FFault,
DblGoTo[MC2ErARet,MC2ErA,R<0], At[MC12,7];*MC2A
*ReadPipe[PipeReg,,FFault], ResetMemErrs,
*DblGoTo[MC2ErBRet,MC2ErB,R<0], At[MC12,10];*MC2B & MC1A/B
ReadPipe[PipeReg,,FFault], ResetMemErrs,
DblGoTo[MC2ErBRet,MC2ErB,R<0], At[MC12,11];*MC2B & MC1A
*ReadPipe[PipeReg,,FFault], ResetMemErrs,
*DblGoTo[MC2ErBRet,MC2ErB,R<0], At[MC12,12];*MC2B & MC1B
ReadPipe[PipeReg,,FFault], ResetMemErrs,
DblGoTo[MC2ErBRet,MC2ErB,R<0], At[MC12,13];*MC2B
*ResetMemErrs, FFault,
* DblGoTo[MC1Notify,MC1Die,R Odd], At[MC12,14];*MC1A/B
ResetMemErrs, FFault,
DblGoTo[MC1Notify,MC1Die,R Odd], At[MC12,15];*MC1A
ReadPipe[PipeReg,,FFault], ResetMemErrs,
DblGoTo[MC1Notify,MC1Die,R Odd], At[MC12,16];*MC1B
T ← MCNoneCrash, GoTo[Crash], At[MC12,17];*None

MC22Die:
T ← MC22Crash, GoTo[Crash];
MC1Die:T ← MC1Crash, GoTo[PipeTaskCrash];

%Set FaultParm to indicate cause of trap:
-1 => Memory Out of Bounds
negative => write protect
positive => page fault
Reference type put in NSALUF for fault handling later.
NOTE: must not attempt to leave the fault results in PipeRegx because
another fault (H4PE or LogSE) might clobber PipeRegx when an io task runs
prior to servicing this fault.
%

MC1Notify:
T ← (PipeReg) and (177C);*Low 7 bits of VPage
*(High 7 bits of VPage’ or low 7 bits)’; bits 0 and 1 wind up both 1.
PipeReg1 ← T ← (LSh[PipeReg1,7]) xnor T;
LU ← LdF[PipeReg5,12,1];*Test dirty’: 0=> page fault
FaultParm ← T, Skip[ALU#0];
FaultParm ← LdF[FaultParm,2,16], Skip;*page fault
*Bits 0 and 1 in FaultParm are both 1 after the xnor and we want them to
*be 2 so that VPage = 37777b won’t be confused with an MOB error.
FaultParm ← (FaultParm) and not (40000C);*write protect fault
MFault:LU ← LdF[RXCTask,0,4];
T ← (PipeReg2) or not (17C), DblGoTo[TestTA,TestTB,ALU#0];

MOBTrap:*Set trap parameter for MOB
FaultParm ← (Zero) - 1, GoTo[MFault];

:UNLESS[LispMode]; *********************************
StackTrap:
LU ← LdF[RXCTask,0,4];
T ← Xor[StkCrash!,377]C, DblGoTo[TestTA,TestTB,ALU#0];
:ENDIF; ********************************************

%Notify emulator at EmNotifyA if emulator was interrupted, else at EmNotifyB.
SALUF is used later at CheckStackTrap to distinguish Stack errors from
others and in other fault handling to give RefType.

Time from location 1 through this non-tasking return:
45 or 46 cycles on MOB;
51 or 52 cycles on Stack overflow or underflow;
55 to 63 cycles on page fault;
56 to 64 cycles on write protect fault.
%
TestTB:RTMP ← LoA[EmuNotifyLoc], Skip;
TestTA:RTMP ← LoA[NonEmuNotifyLoc];
APCTask&APC ← RTMP;
SALUF ← T, Return;

:IF[WithMidas]; *********************************
MC2ErARet:
Return;
MC2ErBRet:
Return;

MC2ErA:T ← LHMask[MemSyndrome], Skip;
MC2ErB:T ← LSh[MemSyndrome,10];
PipeReg ← (RHMask[PipeReg]) or T;
T ← MC2Crash, GoTo[PipeTaskCrash];
:ELSE; ******************************************
MC2ErARet:
MC2ErBRet:
Return;
MC2ErA:
MC2ErB:GoTo[T17RestoreA];
:ENDIF; *****************************************

%T17CheckAPC resumes an interrupted non-emulator task after the emulator’s
TPC has been changed; T17RestoreA and T17RestoreB are the entries used when
the emulator’s TPC has not been changed.

Because APC is used not only for tasking and returning but also for
APCTask&APC← and for dispatches, APC must sometimes be restored to the value
saved at the onset of the trap--otherwise, the mi after a Dispatch or
APCTask&APC← would continue incorrectly. However, if the mi being continued
contains a Return, and if APCTask=0, then control will go to the restored APC
rather than the emulator’s new TPC. Consequently, if APCTask=0, the code
below checks the mi at the continue address for a Return; if so, APC is
changed to the emulator’s new TPC; if not, APC is restored to its trap value.

Delay from location 1 through the Restore below has been:
44 cycles on H4PE;
72 to 82 cycles on Stack overflow or underflow;
73 to 82 cycles on MOB;
83 to 99 cycles on page fault;
84 to 100 cycles on write protect fault.
%
T17CheckAPC:
LU ← LdF[RXAPC,0,4], At[ContNonEmuLoc]; *test saved APCTask
*Only the two low-order bits of T are significant for ReadCS.
T ← 5C, GoTo[T17RestoreA,ALU#0];
*Saved APC is for emulator, must read aborted mi.
APCTask&APC ← RXCTask;
ReadCS;
*This mi need not be at an even mi because task 17b’s TPC is unimportant.
LU ← (LdF[CSData,6,3]) - T - 1;*(JC field) - 6; Return = 6
T ← LdF[RXCTask,4,4], GoTo[T17RestoreB,ALU#0];
RXAPC ← LoA[NonEmuPFLoc], GoTo[T17RestoreB]; *aborted mi does Return.

%Restoring control to a task, as done here, is safe against every type of
interruption except between LoadPage and the subsequent mi or between a
reference and the next mi using the bypass kludge. However, the error
check here will crash distinctively for a LoadPage problem, and since
a following reference is aborted if a page or write protect fault is about
to happen for a PREVIOUS reference, intervention between a reference and the
bypass kludge should only be possible in one of the following three
situations:
1) A Preceding PFetch4 experiences error correction with all 8 cycles of
suspension for its transport occurring between a reference and the next mi
using the bypass kludge, and that reference itself page faults; but page
faults by io tasks aren’t allowed, so this won’t happen.
2) Correctable error logging occurs between a reference and the bypass
kludge (we don’t ever use LogSE).
3) An H4PE intervenes between a reference and the bypass kludge; this is
illegal except for the 3mb Ethernet where enough Nop’s after each Input and
IOStore4 ensure that any H4PE happens safely.
%
T17RestoreA:
T ← LdF[RXCTask,4,4];*Compare page bits
T17RestoreB:
LU ← (LdF[RXPPB,4,4]) xor T;*with saved page register
T ← RXALU, Skip[ALU=0];*result register
T ← LPCrash, GoTo[CTaskCrash];*LoadPage error
APCTask&APC ← RXCTask;
Return, Restore, A ← RXAPC, LU ← T, NoRegILockOK; *back to faulted Task


SetTask[0];

NotifyBack:
xBuf1 ← (xBuf1) or (HiA[ContNonEmuLoc,17]);
APCTask&APC ← xBuf1, xBuf1 ← T, NoRegILockOK;
*GoTo[PFExit] by MesaRefill (MesaJ.Mc) and by PNIP (Initialize.Mc).
PFExit:Return;

EmNotifyB:
UseCTask, xBuf ← T, At[NonEmuNotifyLoc];
T ← APCTask&APC;
*Prepare to notify back to Task 17, location T17CheckAPC
xBuf1 ← LoA[ContNonEmuLoc], Call[NotifyBack];
*PF handling starts here if non-emulator was interrupted.
*If page fault timing is not good enough to prevent UTVFC data lates in the
*worst case, tasking quickly here will help avoid data lates 2/3 of the time,
*so the next two calls are brief (11 cycles and 4 cycles).
T ← StkCrash, Call[CheckStackTrap], At[NonEmuPFLoc];
*xBuf2 ← Saved CIA and restore StkP.
T ← (Stack) and not (170000C), Call[StkPExch];
*Cannot be buffer refill trap or location 0 abort
GoTo[CheckMemStat];

%SALUF holds either [PipeReg2 u 360b] (on a page, write protect, or MOB fault)
or StkCrash’ (stack overflow (StkP=17b) or underflow (StkP=0)). NSALUF then
reads either RefType or StkCrash. First check for a stack error; if not,
save StkP in xBuf2 and point StkP at task 17’s RXCTask register; the caller
will then save CIA of the aborted mi and restore StkP. This is done in two
short subroutines rather than one longer one so io tasks which may have
fallen behind during the long page fault service will have a better
opportunity to catch up.
%
:IF[LispMode]; *************************************
CheckStackTrap:
xBuf2 ← IP[RXCTask]C;
:ELSE; *********************************************
CheckStackTrap:
LU ← (NSALUF) xor T;
xBuf2 ← IP[RXCTask]C, GoTo[StackErrorz,ALU=0];
:ENDIF; ********************************************
T ← (SStkP&NStkP) xor (377C);
StkPExch:
StkP ← xBuf2, xBuf2 ← T, NoRegILockOK, Return;

:UNLESS[LispMode]; *********************************
*We have a stack error. Cause the trap immediately. Restore StkP to
*the value in SStkP saved at the beginning of the opcode.
***Let the PC fall where it may.
StackErrorz:*test SStkP for MaxStack+1 or more
LU ← (SStkP&NStkP) - (LShift[Add[MaxStack!,1],10]C);
T ← SStkP, Skip[Carry’];
T ← MaxStack;
xBuf2 ← T, LoadPage[opPage3];
StkP ← xBuf2;
OnPage[opPage3];
T ← sStackError, GoTo[kfcr];
:ENDIF; ********************************************

EmNotifyA:
UseCTask, xBuf ← T, At[EmuNotifyLoc];*xBuf ← emulator’s T
%FINALLY, task after the following delays:
51 or 52 cycles on emulator MOB;
57 or 58 cycles on stack overflow/underflow;
61 to 68 cycles on page fault;
62 to 69 cycles on write protect fault.
%
T ← APCTask&APC, Task;*xBuf1 ← emulator’s TPC and Task
xBuf1 ← T;
*PF handling starts here if the emulator was interrupted. Note that if the
*emulator was NOT interrupted, then the fault cannot have come from buffer
*refill. Since control entered here, the emulator’s PC is in RXCTask.
T ← StkCrash, Call[CheckStackTrap];
T ← (Stack) and not (170000C), Call[StkPExch];
%At this point we have:
xBufemulator’s T at the time of the fault;
xBuf1emulator’s TPC;
xBuf2CIA of the aborted mi.
The major problem with faults is to determine the PC to store in the frame,
and whether to continue the opcode, or cause the trap immediately.
The cases are:

1) The fault was detected at location 0, the 1st mi of the buffer refill
trap. In this case, buffer refill was just starting when some previous
reference faulted. Treat this as in case 7 below.

2) A PFetch4 fault occurred on page 0 and the emulator’s TPC points at the 1st
mi of a bytecode (TPC = 01xxxxxxxx01). It is assumed that the fault is due to
a NextData/NextInst buffer refill; SStkP and PCX do not yet reflect advance
from the previous bytecode, so the trap is started with PC = 2*PCB - 1
(NextData/NextInst was accessing an operand from location 0 of the buffer, so
the opcode was byte 7 of the previous buffer, but PCB ← (PCB) + (4C) occurred
before the fault) and with the current value of StkP rather than SStkP.

3) A PFetch4 fault occurred on page 0 and the emulator’s TPC points elsewhere
than as in case 2. In this case, it is assumed that the fault is due to a
NextData or NextInst buffer refill which are differentiated by the low bit
of F1 in the mi pointed at by the emulator’s TPC. If the fault is due to a
NextData, nothing special is done, but for a NextInst, IBuf to IBuf3 are
set to -1, PCF to 0, and control is sent to the NextInst so other work done
by the mi containing the NextInst and the mi after that can be completed
before the trap. Control then goes to opcode 377, which will cause the
trap with PC = 2*PCB + PCX - 1.

4) A PFetch4 fault occurred on page 6. This is a jump opcode buffer refill
handled as in case 3 without zeroing PCF.

5) The fault was due to an Xfer buffer refill (MemStat[13:15] =
XferFixup). This is handled just like a jump (case 4).

6) The fault occured during the early phases of Xfer. We want to back
out and redo the opcode, but CODE may have changed and we need it
to compute the PC to save. Call Loadc to reload from the current LOCAL.

7) If none of these situations hold, the PC is (PCB*2) + q, where q = if
(PCF>=PCX) then PCX-1 else PCX-9 (if PCF<PCX, then the buffer was
refilled between the NextInst and the fault, and PCB has been advanced
by 8 bytes). In the normal case, the trap is started using this PC, and
it is not necessary to unwind the instruction. If any special unwinding
is necessary,it is indicated by a value in MemStat[13:15].
%
T ← 6C;
LU ← LdF[xBuf2,4,4];
*Go if emulator page 0 fault; test for RefType=6 (PFetch4)
LU ← (NSALUF) xor T, GoTo[CameFromPage0,ALU=0];
*GoTo CheckMemStat if not a PFetch4.
LU ← (LdF[xBuf2,4,4]) xor T, GoTo[CheckMemStat,ALU#0];
*The operation is a jump buffer refill if it is a PFetch4 from page 6.
DblGoTo[CMSt1,ResumeBytecode,ALU#0];


CameFromPage0:
LU ← xBuf2, GoTo[CMSt2,ALU#0];*test for aborted CIA = 0
*The operation is a PFetch4 from page 0--ASSUME it is buffer refill.
*Test for emulator TPC = 01xxxxxxxx01; put the high 2 and low 2 bits of TPC
*in the right-half with task 0 in the middle.
xBuf1 ← LCy[xBuf1,6], GoTo[CMSt3,ALU=0];
T ← 101C;
LU ← (RHMask[xBuf1]) xor T;
T ← RCy[xBuf1,6], GoTo[CIB1st,ALU=0];*T ← original xBuf1
*Fault was NOT from 1st mi of bytecode; find out whether emulator’s TPC
*points at a NextInst or NextData.
xBuf2 ← T;*xBuf2 ← emulator’s TPC instead of aborted CIA
T ← RZero;*Does TPC point to a NextInst?
APCTask&APC ← xBuf2;
ReadCS;
*Branch on probable NextData; fall through on probable NextInst.
CSData, GoTo[CMSt0,R Odd];
PCF ← RZero; *PCF ← 0
%Jump to ResumeByteCode on a PFetch4 from page 6 (Mesa jumps all on page 6).
Continue the jump after filling IBuf with -1, and eventually get to
opcode 377, which will start the trap.
%
ResumeBytecode:
T ← xBuf;*Restore emulator’s T register

%Jump to CIB1 on XferFixup (trap at the end of Xfer after PFetch4 to refill
IBuf has been launched).
***Try a single PCF[IBuf]← here when everything seems solid.
%
CIB1:IBuf ← (Zero) - 1, Task;
IBuf1 ← (Zero) - 1;
IBuf2 ← (Zero) - 1;
APCTask&APC ← xBuf2;*Return to the NextInst or whatever
IBuf3 ← (Zero) - 1, Return;*Force bytecode 377


:IF[LispMode]; *************************************

*TPC = 01xxxxxxxx01, fault was from refill for NextData or NextInst in 1st mi
*of bytecode
CIB1st:LoadPageExternal[7];
CallExternal[3761];*Execute opcode 374 (return);

CMSt3:GoTo[MemFault];*PFetch4 from location 0
CMSt2:GoTo[MemFault];*Not a PFetch4, fault on pg 0
CMSt1:GoTo[MemFault];*PFetch4 not on pp 0 or 6
CMSt0:GoTo[MemFault];*NextData fault, not 1st mi of bytecode
CheckMemStat:
GoTo[MemFault];*Non-emu fault or non-page 0 or 6 fault

FPCOx:GoTo[MemFault];*Regular fault on page 0

:IFE[Add[IP[PipeReg4],3], IP[FaultParm],, ER[PipeReg4.and.FaultParm.changed]];

MemFault:
T ← SStkP;* Get saved StkP
xBuf2 ← IP[FaultParm]C, Call[StkPExch];
T ← Stack&-3;* T ← FaultParm; Stkp points at PipeReg4
AC0 ← T, GoTo[lspMapFaultPunt];

:ELSE; *********************************************
*TPC = 01xxxxxxxx01, fault was from refill for NextData or NextInst in 1st mi
*of bytecode.
CIB1st:
PCB ← (PCB) - (4C);
PCF ← AllOnes;*PCF ← 7
T ← (SStkP&NStkP) xor (377C), GoTo[SMTrpx];

CMSt3:Dispatch[MemStat,15,3], GoTo[CMStD];*PFetch4 fault at location 0.
CMSt2:Dispatch[MemStat,15,3], GoTo[CMStD];*Not a PFetch4, fault on pg 0.
CMSt1:Dispatch[MemStat,15,3], GoTo[CMStD];*PFetch4 not on pp 0 or 6.
CMSt0:Dispatch[MemStat,15,3], GoTo[CMStD];*NextData refill fault, not
*1st mi of bytecode.
CheckMemStat:
Dispatch[MemStat,15,3];*Non-emu fault or not a PFetch4, fault on pg#0
CMStD:Disp[FixPCOnly];


*The fault was due to an Xfer buffer refill (MemStat[13:15] = XferFixup).
*Handle just like a jump.
FixXfer:
T ← xBuf, GoTo[CIB1], At[FixDisp,XferFixup!];

*The fault occured during the early phases of Xfer. We want to back out and
*redo the bytecode, but CODE may have changed and we need it to compute the PC
*to save. Fetch G and the overhead, and call LoadC to reload from the
*current LOCAL.
FixEarlyXfer:
PFetch1[LOCAL,GLOBAL,0], At[FixDisp,EarlyXfer!];
Call[FP1Ret];
T ← GLOBAL, LoadPage[xfPage1];
PFetch4[MDS,IBuf], Call[LoadC];*Fetch the global frame overhead
xfGFIWord ← T, GoTo[FixPCOnly];


*Prepare for fixup relative to SStkP.
FixBLTL:
T ← SStkP, At[FixDisp,BltLFixup!];
RTemp ← T;
RTemp ← (RTemp) - (4C), Call[FSetStkP];
T ← xBuf, Call[BumpGlorp];*source + T
Stack&+1, Call[FBumpStk];*count + 1
Stack&+1, Call[BumpGlorp];*dest + T
FPCOz:T ← (PCXReg) - 1, GoTo[FPCOx];*one byte inst cannot have refilled buffer

BumpGlorp:
Stack ← (Stack) + T;
Stack&+1, Skip[Carry’];
FBumpStk:
Stack ← (Stack) + 1, Return;
FP1Ret:Return;

FSetStkP:
StkP ← RTemp, RTemp ← T, NoRegILockOK, Return;


*Prepare for fixup relative to SStkP.
FixBlt:T ← (SStkP) - 1, At[FixDisp,BltFixup!];
*Stack points at operation
RTemp ← T;
StkP ← RTemp;*Point StkP at Count
*Operation in low 4 bits of NSALUF was PFetch1 (type 4) or PStore1 (type 10b)
T ← LdF[ALUResult&NSALUF,14,1];*1=> store, 0=> fetch
*Count+1; if fetch, done with fixup.
Stack ← (Stack) + 1, GoTo[FPCOz,ALU=0];
Stack&-1, Call[DecGlorp];*Source - 1
Stack&+2, Call[DecGlorp];*Dest - 1
T ← (PCXReg) - 1, GoTo[FPCOx];

DecGlorp:
Stack ← (Stack) - 1, Return;

FixPCOnly:
T ← (PCXReg) - 1, At[FixDisp,Normal!]; *normal PC fixup
FPCOx:LU ← (PCFreg) - T, Skip[ALU>=0];
T ← 7C, GoTo[.-1];*PCX was 0; opcode started in prev. quadword
*If PCF .eq. 7 then we already backed up PCB,
*so don’t subtract 4 from PCB again.
RTemp ← T, Skip[ALU>=0];
PCB ← (PCB) - (4C);*PCX large, PCF small = > prev. quadword
PCF ← RTemp;*PCF is always PCX-1, only PCB is in doubt

%FaultParm has been set up by the Trap handler:
-1 => Map Out of Bounds
negative => write protect, page in low 14 bits
positive => page fault, page in low 14 bits
Here PCB,PCF is correct pc to save for trap. It will be done through KFCB.
%
StartMemTrap:
T ← SStkP;
SMTrpx:RTemp ← IP[FaultParm]C, Call[FSetStkP];
T ← (Stack) and not (140000C);
LU ← (Stack) + 1;
xfOTPReg ← T, Skip[ALU>=0];
T ← sWriteProtect, Skip;*Write protect fault
T ← sPageFault;*Page fault or MOB
LoadPage[opPage3];
StkP ← RTemp, GoToP[kfcr];
:ENDIF; ********************************************

:END[Fault];