171 lines
5.3 KiB
Text
171 lines
5.3 KiB
Text
.SN 9
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.VS 1 0
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.BP
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.S1 "TRAPS AND INTERRUPTS"
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EM provides a means for the user program to catch all traps
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generated by the program itself, the hardware, or external conditions.
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This mechanism uses five instructions: LIM, SIM, SIG, TRP and RTT.
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This section of the manual may be omitted on the first reading since it
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presupposes knowledge of the EM instruction set.
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.P
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The action taken when a trap occures is determined by the value
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of an internal EM trap register.
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This register contains a pointer to a procedure.
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Initially the pointer used is zero and all traps halt the
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program with, hopefully, a useful message to the outside world.
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The SIG instruction can be used to alter the trap register,
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it pops a procedure pointer from the
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stack into the trap register.
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When a trap occurs after storing a nonzero value in the trap
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register, the procedure pointed to by the trap register
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is called with the trap number
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as the only parameter (see below).
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SIG returns the previous value of the trap register on the
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stack.
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Two consecutive SIGs are a no-op.
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When a trap occurs, the trap register is reset to its initial
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condition, to prevent recursive traps from hanging the machine up,
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e.g. stack overflow in the stack overflow handling procedure.
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.P
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The runtime systems for some languages need to ignore some EM
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traps.
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EM offers a feature called the ignore mask.
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It contains one bit for each of the lowest 16 trap numbers.
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The bits are numbered 0 to 15, with the least significant bit
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having number 0.
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If a certain bit is 1 the corresponding trap never
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occurs and processing simply continues.
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The actions performed by the offending instruction are
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described by the Pascal program in appendix A.
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.N
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If the bit is 0, traps are not ignored.
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The instructions LIM and SIM allow copying and replacement of
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the ignore mask.~
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.P
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The TRP instruction generates a trap, the trap number being found on the
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stack.
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This is, among other things,
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useful for library procedures and runtime systems.
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It can also be used by a low level trap procedure to pass the trap to a
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higher level one (see example below).
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.P
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The RTT instruction returns from the trap procedure and continues after the
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trap.
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In the list below all traps marked with an asterisk ('*') are
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considered to be fatal and it is explicitly undefined what happens if
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you try to restart after the trap.
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.P
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The way a trap procedure is called is completely compatible
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with normal calling conventions. The only way a trap procedure
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differs from normal procedures is the return. It has to use RTT instead
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of RET. This is necessary because the complete runtime status is saved on the
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stack before calling the procedure and all this status has to be reloaded.
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Error numbers are in the range 0 to 252.
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The trap numbers are divided into three categories:
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.IS 4
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.N 1
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.PS - 10
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.PT ~~0\-~63
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EM machine errors, e.g. illegal instruction.
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.PS - 8
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.PT ~0\-15
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maskable
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.PT 16\-63
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not maskable
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.PE
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.PT ~64\-127
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Reserved for use by compilers, run time systems, etc.
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.PT 128\-252
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Available for user programs.
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.PE 1
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.IE
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EM machine errors are numbered as follows:
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.DS I 5
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.TS
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tab(@);
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n l l.
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0@EARRAY@Array bound error
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1@ERANGE@Range bound error
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2@ESET@Set bound error
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3@EIOVFL@Integer overflow
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4@EFOVFL@Floating overflow
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5@EFUNFL@Floating underflow
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6@EIDIVZ@Divide by 0
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7@EFDIVZ@Divide by 0.0
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8@EIUND@Undefined integer
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9@EFUND@Undefined float
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10@ECONV@Conversion error
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16*@ESTACK@Stack overflow
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17@EHEAP@Heap overflow
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18*@EILLINS@Illegal instruction
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19*@EODDZ@Illegal size argument
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20*@ECASE@Case error
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21*@EMEMFLT@Addressing non existent memory
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22*@EBADPTR@Bad pointer used
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23*@EBADPC@Program counter out of range
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24@EBADLAE@Bad argument of LAE
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25@EBADMON@Bad monitor call
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26@EBADLIN@Argument of LIN too high
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27@EBADGTO@GTO descriptor error
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.TE
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.DE 0
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.P
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As an example,
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suppose a subprocedure has to be written to do a numeric
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calculation.
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When an overflow occurs the computation has to be stopped and
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the higher level procedure must be resumed.
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This can be programmed as follows using the mechanism described above:
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.DS B
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.ta 1n 24n
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mes 2,2,2 ; set sizes
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ersave
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bss 2,0,0 ; Room to save previous value of trap procedure
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msave
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bss 2,0,0 ; Room to save previous value of trap mask
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pro calcule,0 ; entry point
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lxl 0 ; fill in non-local goto descriptor with LB
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ste jmpbuf+4
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lor 1 ; and SP
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ste jmpbuf+2
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lim ; get current ignore mask
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ste msave ; save it
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lim
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loc 16 ; bit for EFOVFL
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ior 2 ; set in mask
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sim ; ignore EFOVFL from now on
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lpi $catch ; load procedure identifier
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sig ; catch wil get all traps now
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ste ersave ; save previous trap procedure identifier
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; perform calculation now, possibly generating overflow
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1 ; label jumped to by catch procedure
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loe ersave ; get old trap procedure
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sig ; refer all following trap to old procedure
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asp 2 ; remove result of sig
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loe msave ; restore previous mask
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sim ; done now
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; load result of calculation
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ret 2 ; return result
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jmpbuf
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con *1,0,0
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end
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.DE 0
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.VS 1 1
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.DS
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Example of catch procedure
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.ta 1n 24n
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pro catch,0 ; Local procedure that must catch the overflow trap
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lol 2 ; Load trap number
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loc 4 ; check for overflow
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bne *1 ; if other trap, call higher trap procedure
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gto jmpbuf ; return to procedure calcule
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1 ; other trap has occurred
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loe ersave ; previous trap procedure
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sig ; other procedure will get the traps now
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asp 2 ; remove the result of sig
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lol 2 ; stack trap number
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trp ; call other trap procedure
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rtt ; if other procedure returns, do the same
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end
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.DE
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