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.TL
The UNIX APL Interpreter -- UCSF Version
.AU
H. Ross Harvey
.AI
UCSF Computer Graphics Laboratory
.AB
The UNIX APL interpreter is an incremental `compiler' which
employs a multi-pass translator to produce an intermediate
code. This technique is much faster
than the normal interpretive method and requires less memory.
The action of the translator is visible
in only one way: poor run-time error reporting.
The UCSF APL version features a 50K byte workspace, a simple
file access procedure, character comparisons and an extended
set of I-beams. The interpreter itself has been made much more
reliable and the error messages have been improved.
.AE
.ND
.PP
UNIX APL is a very well written package containing all the
APL 360 operators plus execute, scan, and relational character
operators. UNIX APL does not have a trace feature or a state
indicator.
.NH 1
Major Data Structures
.NH 2
Overview
.PP
Functions are compiled when they are first referenced;
this compiled code is then stored in memory. Data always
resides in memory, although there is a facility for
reading and write files. The source
code to the functions is maintained in the disk-saved workspaces,
in the APL temporary file, and possibly as single user files.
The generated code usually consists of single-byte values
which are indices into the array \fBexop\fR.
This array contains the addresses of functions implementing the
APL operators. A reference to a variable will be specified by the
\fBNAME\fR
operator and the two-byte address of an \fBitem\fR or \fBnlist\fR
structure. A reference to a constant will be specified by the
\fBELID\fR or \fBCONST\fR operator and an eight-byte double constant.
.NH 2
The Item and Nlist Structures
.PP
The item and nlist structures are:
.DS
.B
struct item {		    struct	nlist {
	char	rank;		    char	use;
	char	type;		    char	type;
	int	size;		    int	*itemp;
	int	index;		    char	*namep;
	data	*datap;		    int	label;
	int	dim[0];	    };
};
.R
.DE
The \fBrank\fR element is precisly the APL rank of the variable.
The \fBtype\fR element may contain one status byte.
The \fBuse\fR element contains the same information as the type
element of the item structure. \fBItemp\fR usually points to
the item structure describing a variable or function whose name
is addressed by the \fBnamep\fR element.
If the nlist structure describes a function, then itemp will
be zero until the function is referenced and compiled; if
the nlist structure describes some other type of variable, itemp
will be zero until the variable has been set.
\fBSize\fR is the total number
of elements in the vector and index is an index into the
currently selected member. \fBDatap\fR points to the actual data
and dim[] (actually dim[rank]) naturally contains the various
dimensions of the object.
Since the \fBtype\fR field
indicates important information such as whether the data
can be destroyed, it is important to understand the
meaning of the different types.
.IP DA 10
This indicates numeric data \fInot\fR associated with
some variable. This attribute is given to data which
exists only on the stack in some way as an intermediate
value. Objects of type \fBDA\fR may be overwritten if necessary
and \fBwill\fR be deallocated if they are found on the
stack after a line is executed.
.IP LV
\fBLV\fR indicates an \fBlvalue\fR or assignable quantity. This descriptor
declares that the element is the actual data of some variable. It
can not be overwritten or de-allocated. It also specifies that
the pointed-to structure is not an item structure at all
but is in fact the nlist structure. In this case, the
\fBuse\fR member will specify the actual data type
and the \fBitemp\fR member will point to an \fBitem\fR structure.
.IP CH
This indicates character data.
.IP QD
This is the \fBquad\fR variable. A reference to a quantity of this
type will cause the appropriate processing (an expression is read
and evaluated) and result in an element of type \fBDA\fR being placed
on the stack.
.IP QQ
\fBQQ\fR refers to a quote-quad variable. A line of text is read from the
terminal and placed (as type \fBCH\fR) on the stack.
.IP IN
This is integer data. This type is not fully supported so it
works only when the next operator is expecting an integer.
.IP EL
This data type is used for data entered literally in function
calls. \fBEL\fR data is deallocated in the UCSF version.
.IP "NF MF DF"
These types specify functions which are to be called with
0, 1 or 2 arguments.
.NH 2
The Env Structure
.PP
The \fBenv\fR structure contains the index origin, printing
precision, terminal width, and fuzz factor associated with the
current workspace. When a saved workspace is loaded, these
parameters are restored to their state at the time the workspace
was saved.
.NH 2
The Nlist Array
.PP
The \fBnlist\fR structure described above is found in an
array of structures which is also called \fBnlist\fR.
This array is maintained by ???????.
.NH 2
The Labldefs Structure
.PP
The \fBlabldefs\fR structure is the start of a linked list
containing label-name/line-number pairs. This list is allocated
dynamically and is used only when a function is being compiled.
.NH 2
The Idx Structure
.PP
The \fBidx\fR structure is used ????????.
.NH 2
The Stack
.PP
Most operators take their source operands from the top
of the internal stack. Most operators place their
results on the top of the stack. In addition, local
variables require space on the stack at each entrance
to the function with which they are bound.
The principle objects
involved here are:
.DS
.I
struct item **sp, **stack, **staktop;
.R
.DE
The address of a memory block of size \fIsizeof(sp)*STKS\fR is
initially assigned to \fBstack\fR. The variable \fBsp\fR
points the the top of the stack.
A call to the machine-coded function \fBpush(\fIaddress\fB)\fR
will place \fIaddress\fR on top of the stack, incrementing
\fBsp\fR appropriatly. \fBStaktop\fR marks the top of the
current stack; if \fBpush(...)\fR finds that \fBsp\fR
has passed \fBstaktop\fR, a call to \fBnewstak()\fR will
allocate a new (contiguous) stack that is \fBSTKS\fR words
larger. The information on the old stack is copied to the
new one; \fBstack\fR is assigned the address of the new
stack, and the memory occupied by the old stack is freed.
.PP
The function \fBReset\fR is called when APL returns
to the top command level or when any error is detected.
\fBReset\fR frees the current stack and allocates a
new stack of size \fBSTKS\fR. This is done to ensure
the integrity of the stack in the face of errors such
as \fIWS EXCEEDED\fR or \fBINTERRUPT\fR. In addition,
recursive or deeply nested function calls
will cause a large amount of memory to be allocated to
the stack. It is considered desireable to reset the stack
to a small default value when possible. This prevents
intentional or accidental recursion from impairing
the operation of the interpreter by permanently allocating
a large block of memory.