164 lines
5.8 KiB
Plaintext
164 lines
5.8 KiB
Plaintext
.SN 7
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.BP
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.S1 "DESCRIPTORS"
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Several instructions use descriptors, notably the range check instruction,
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the array instructions, the goto instruction and the case jump instructions.
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Descriptors reside in data space.
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They may be constructed at run time, but
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more often they are fixed and allocated in ROM data.
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.P
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All instructions using descriptors, except GTO, have as argument
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the size of the integers in the descriptor.
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All implementations have to allow integers of the size of a
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word in descriptors.
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All integers popped from the stack and used for indexing or comparing
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must have the same size as the integers in the descriptor.
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.S2 "Range check descriptors"
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Range check descriptors consist of two integers:
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.IS 2
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.PS 1 4 "" .
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.PT
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lower bound~~~~~~~signed
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.PT
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upper bound~~~~~~~signed
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.PE
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.IE
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The range check instruction checks an integer on the stack against
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these bounds and causes a trap if the value is outside the interval.
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The value itself is neither changed nor removed from the stack.
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.S2 "Array descriptors"
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Each array descriptor describes a single dimension.
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For multi-dimensional arrays, several array instructions are
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needed to access a single element.
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Array descriptors contain the following three integers:
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.IS 2
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.PS 1 4 "" .
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.PT
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lower bound~~~~~~~~~~~~~~~~~~~~~signed
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.PT
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upper bound - lower bound~~~~~~~unsigned
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.PT
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number of bytes per element~~~~~unsigned
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.PE
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.IE
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The array instructions LAR, SAR and AAR have the pointer to the start
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of the descriptor as operand on the stack.
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.sp
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The element A[I] is fetched as follows:
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.IS 2
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.PS 1 4 "" .
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.PT
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Stack the address of A (e.g., using LAE or LAL)
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.PT
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Stack the value of I (n-byte integer)
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.PT
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Stack the pointer to the descriptor (e.g., using LAE)
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.PT
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LAR n (n is the size of the integers in the descriptor and I)
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.PE
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.IE
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All array instructions first pop the address of the descriptor
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and the index.
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If the index is not within the bounds specified, a trap occurs.
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If ok, (I~-~lower bound) is multiplied
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by the number of bytes per element (the third word). The result is added
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to the address of A and replaces A on the stack.
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.A
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At this point LAR, SAR and AAR diverge.
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AAR is finished. LAR pops the address and fetches the data
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item,
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the size being specified by the descriptor.
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The usual restrictions for memory access must be obeyed.
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SAR pops the address and stores the
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data item now exposed.
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.S2 "Non-local goto descriptors"
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The GTO instruction provides a way of returning directly to any
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active procedure invocation.
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The argument of the instruction is the address of a descriptor
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containing three pointers:
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.IS 2
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.PS 1 4 "" .
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.PT
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value of PC after the jump
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.PT
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value of SP after the jump
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.PT
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value of LB after the jump
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.PE
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.IE
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GTO replaces the loads PC, SP and LB from the descriptor,
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thereby jumping to a procedure
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and removing zeor or more frames from the stack.
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The LB, SP and PC in the descriptor must belong to a
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dynamically enclosing procedure,
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because some EM implementations will need to backtrack through
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the dynamic chain and use the implementation dependent data
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in frames to restore registers etc.
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.S2 "Case descriptors"
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The case jump instructions CSA and CSB both
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provide multiway branches selected by a case index.
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Both fetch two operands from the stack:
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first a pointer to the low address of the case descriptor
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and then the case index.
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CSA uses the case index as index in the descriptor table, but CSB searches
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the table for an occurrence of the case index.
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Therefore, the descriptors for CSA and CSB,
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as shown in figure 4, are different.
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All pointers in the table must be addresses of instructions in the
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procedure executing the case instruction.
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.P
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CSA selects the new PC by indexing.
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If the index, a signed integer, is greater than or equal to
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the lower bound and less than or equal to the upper bound,
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then fetch the new PC from the list of instruction pointers by indexing with
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index-lower.
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The table does not contain the value of the upper bound,
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but the value of upper-lower as an unsigned integer.
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The default instruction pointer is used when the index is out of bounds.
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If the resulting PC is 0, then trap.
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.P
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CSB selects the new PC by searching.
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The table is searched for an entry with index value equal to the case index.
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That entry or, if none is found, the default entry contains the
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new PC.
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When the resulting PC is 0, a trap is performed.
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.P
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The choice of which case instruction to use for
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each source language case statement
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is up to the front end.
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If the range of the index value is dense, i.e
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.DS
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(highest value - lowest value) / number of cases
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.DE 1
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is less than some threshold, then CSA is the obvious choice.
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If the range is sparse, CSB is better.
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.N 2
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.DS
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|--------------------| |--------------------| high address
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| pointer for upb | | pointer n-1 |
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|--------------------| |- - - - - - - |
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| . | | index n-1 |
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| . | |--------------------|
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| . | | . |
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| . | | . |
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| . | | . |
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| . | |--------------------|
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| . | | pointer 1 |
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|--------------------| |- - - - - - - |
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| pointer for lwb+1 | | index 1 |
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|--------------------| |--------------------|
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| pointer for lwb | | pointer 0 |
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|--------------------| |- - - - - - - |
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| upper - lower | | index 0 |
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|--------------------| |--------------------|
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| lower bound | | number of entries |
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|--------------------| |--------------------|
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| default pointer | | default pointer | low address
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|--------------------| |--------------------|
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CSA descriptor CSB descriptor
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Figure 4. Descriptor layout for CSA and CSB
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.DE
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