62 lines
2.1 KiB
Plaintext
62 lines
2.1 KiB
Plaintext
.SN 3
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.BP
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.S1 "INSTRUCTION ADDRESS SPACE"
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The instruction space of the EM machine contains
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the code for procedures.
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Tables necessary for the execution of this code, for example, procedure
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descriptor tables, may also be present.
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The instruction space does not change during
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the execution of a program, so that it may be
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protected.
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No further restrictions to the instruction address space are
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necessary for the abstract and assembly language level.
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.P
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Each procedure has a single entry point: the first instruction.
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A special type of pointer identifies a procedure.
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Pointers into the instruction
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address space have the same size as pointers into data space and
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can, for example, contain the address of the first instruction
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or an index in a procedure descriptor table.
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.A
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There is a single EM program counter, PC, pointing
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to the next instruction to be executed.
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The procedure pointed to by PC is
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called the 'current' procedure.
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A procedure may call another procedure using the CAL or CAI
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instruction.
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The calling procedure remains 'active' and is resumed whenever the called
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procedure returns.
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Note that a procedure has several 'active' invocations when
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called recursively.
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.P
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Each procedure must return properly.
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It is not allowed to fall through to the
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code of the next procedure.
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There are several ways to exit from a procedure:
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.IS 3
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.PS
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.PT
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the RET instruction, which returns to the
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calling procedure.
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.PT
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the RTT instruction, which exits a trap handling routine and resumes
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the trapping instruction (see next chapter).
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.PT
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the GTO instruction, which is used for non-local goto's.
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It can remove several frames from the stack and transfer
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control to an active procedure.
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.PE
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.IE
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.P
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All branch instructions can transfer control
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to any label within the same procedure.
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Branch instructions can never jump out of a procedure.
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.P
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Several language implementations use a so called procedure
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instance identifier, a combination of a procedure identifier and
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the LB of a stack frame, also called static link.
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.P
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The program text for each procedure, as well as any tables,
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are fragments and can be allocated anywhere
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in the instruction address space.
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