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1.1 root 1: .nr PO .5i
2: .de B
3: .ft B
4: ..
5: .RP
6: .TL
7: The UNIX APL Interpreter -- UCSF Version
8: .AU
9: H. Ross Harvey
10: .AI
11: UCSF Computer Graphics Laboratory
12: .AB
13: The UNIX APL interpreter is an incremental `compiler' which
14: employs a multi-pass translator to produce an intermediate
15: code. This technique is much faster
16: than the normal interpretive method and requires less memory.
17: The action of the translator is visible
18: in only one way: poor run-time error reporting.
19: The UCSF APL version features a 50K byte workspace, a simple
20: file access procedure, character comparisons and an extended
21: set of I-beams. The interpreter itself has been made much more
22: reliable and the error messages have been improved.
23: .AE
24: .ND
25: .PP
26: UNIX APL is a very well written package containing all the
27: APL 360 operators plus execute, scan, and relational character
28: operators. UNIX APL does not have a trace feature or a state
29: indicator.
30: .NH 1
31: Major Data Structures
32: .NH 2
33: Overview
34: .PP
35: Functions are compiled when they are first referenced;
36: this compiled code is then stored in memory. Data always
37: resides in memory, although there is a facility for
38: reading and write files. The source
39: code to the functions is maintained in the disk-saved workspaces,
40: in the APL temporary file, and possibly as single user files.
41: The generated code usually consists of single-byte values
42: which are indices into the array \fBexop\fR.
43: This array contains the addresses of functions implementing the
44: APL operators. A reference to a variable will be specified by the
45: \fBNAME\fR
46: operator and the two-byte address of an \fBitem\fR or \fBnlist\fR
47: structure. A reference to a constant will be specified by the
48: \fBELID\fR or \fBCONST\fR operator and an eight-byte double constant.
49: .NH 2
50: The Item and Nlist Structures
51: .PP
52: The item and nlist structures are:
53: .DS
54: .B
55: struct item { struct nlist {
56: char rank; char use;
57: char type; char type;
58: int size; int *itemp;
59: int index; char *namep;
60: data *datap; int label;
61: int dim[0]; };
62: };
63: .R
64: .DE
65: The \fBrank\fR element is precisly the APL rank of the variable.
66: The \fBtype\fR element may contain one status byte.
67: The \fBuse\fR element contains the same information as the type
68: element of the item structure. \fBItemp\fR usually points to
69: the item structure describing a variable or function whose name
70: is addressed by the \fBnamep\fR element.
71: If the nlist structure describes a function, then itemp will
72: be zero until the function is referenced and compiled; if
73: the nlist structure describes some other type of variable, itemp
74: will be zero until the variable has been set.
75: \fBSize\fR is the total number
76: of elements in the vector and index is an index into the
77: currently selected member. \fBDatap\fR points to the actual data
78: and dim[] (actually dim[rank]) naturally contains the various
79: dimensions of the object.
80: Since the \fBtype\fR field
81: indicates important information such as whether the data
82: can be destroyed, it is important to understand the
83: meaning of the different types.
84: .IP DA 10
85: This indicates numeric data \fInot\fR associated with
86: some variable. This attribute is given to data which
87: exists only on the stack in some way as an intermediate
88: value. Objects of type \fBDA\fR may be overwritten if necessary
89: and \fBwill\fR be deallocated if they are found on the
90: stack after a line is executed.
91: .IP LV
92: \fBLV\fR indicates an \fBlvalue\fR or assignable quantity. This descriptor
93: declares that the element is the actual data of some variable. It
94: can not be overwritten or de-allocated. It also specifies that
95: the pointed-to structure is not an item structure at all
96: but is in fact the nlist structure. In this case, the
97: \fBuse\fR member will specify the actual data type
98: and the \fBitemp\fR member will point to an \fBitem\fR structure.
99: .IP CH
100: This indicates character data.
101: .IP QD
102: This is the \fBquad\fR variable. A reference to a quantity of this
103: type will cause the appropriate processing (an expression is read
104: and evaluated) and result in an element of type \fBDA\fR being placed
105: on the stack.
106: .IP QQ
107: \fBQQ\fR refers to a quote-quad variable. A line of text is read from the
108: terminal and placed (as type \fBCH\fR) on the stack.
109: .IP IN
110: This is integer data. This type is not fully supported so it
111: works only when the next operator is expecting an integer.
112: .IP EL
113: This data type is used for data entered literally in function
114: calls. \fBEL\fR data is deallocated in the UCSF version.
115: .IP "NF MF DF"
116: These types specify functions which are to be called with
117: 0, 1 or 2 arguments.
118: .NH 2
119: The Env Structure
120: .PP
121: The \fBenv\fR structure contains the index origin, printing
122: precision, terminal width, and fuzz factor associated with the
123: current workspace. When a saved workspace is loaded, these
124: parameters are restored to their state at the time the workspace
125: was saved.
126: .NH 2
127: The Nlist Array
128: .PP
129: The \fBnlist\fR structure described above is found in an
130: array of structures which is also called \fBnlist\fR.
131: This array is maintained by ???????.
132: .NH 2
133: The Labldefs Structure
134: .PP
135: The \fBlabldefs\fR structure is the start of a linked list
136: containing label-name/line-number pairs. This list is allocated
137: dynamically and is used only when a function is being compiled.
138: .NH 2
139: The Idx Structure
140: .PP
141: The \fBidx\fR structure is used ????????.
142: .NH 2
143: The Stack
144: .PP
145: Most operators take their source operands from the top
146: of the internal stack. Most operators place their
147: results on the top of the stack. In addition, local
148: variables require space on the stack at each entrance
149: to the function with which they are bound.
150: The principle objects
151: involved here are:
152: .DS
153: .I
154: struct item **sp, **stack, **staktop;
155: .R
156: .DE
157: The address of a memory block of size \fIsizeof(sp)*STKS\fR is
158: initially assigned to \fBstack\fR. The variable \fBsp\fR
159: points the the top of the stack.
160: A call to the machine-coded function \fBpush(\fIaddress\fB)\fR
161: will place \fIaddress\fR on top of the stack, incrementing
162: \fBsp\fR appropriatly. \fBStaktop\fR marks the top of the
163: current stack; if \fBpush(...)\fR finds that \fBsp\fR
164: has passed \fBstaktop\fR, a call to \fBnewstak()\fR will
165: allocate a new (contiguous) stack that is \fBSTKS\fR words
166: larger. The information on the old stack is copied to the
167: new one; \fBstack\fR is assigned the address of the new
168: stack, and the memory occupied by the old stack is freed.
169: .PP
170: The function \fBReset\fR is called when APL returns
171: to the top command level or when any error is detected.
172: \fBReset\fR frees the current stack and allocates a
173: new stack of size \fBSTKS\fR. This is done to ensure
174: the integrity of the stack in the face of errors such
175: as \fIWS EXCEEDED\fR or \fBINTERRUPT\fR. In addition,
176: recursive or deeply nested function calls
177: will cause a large amount of memory to be allocated to
178: the stack. It is considered desireable to reset the stack
179: to a small default value when possible. This prevents
180: intentional or accidental recursion from impairing
181: the operation of the interpreter by permanently allocating
182: a large block of memory.
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