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.\"     @(#)pxin1.n     5.2 (Berkeley) 4/17/91
.\" $FreeBSD: src/share/doc/papers/px/pxin1.n,v 1.1.1.1.14.1 2000/11/30 17:13:59 ru Exp $
.\"
.tr _\(ru
.nr H1 0
.NH 
Organization
.PP
Most of
.I px
is written in the
.SM "VAX 11/780"
assembly language, using the
.UX
assembler
.I as.
Portions of
.I px
are also written in the
.UX
systems programming language C.
.I Px
consists of a main procedure that reads in the interpreter code,
a main interpreter loop that transfers successively to various
code segments implementing the abstract machine operations,
built-in procedures and functions,
and several routines that support the implementation of the
Pascal input-output environment.
.PP
The interpreter runs at a fraction of the speed of equivalent
compiled C code, with this fraction varying from 1/5 to 1/15.
The interpreter occupies 18.5K bytes of instruction space, shared among
all processes executing Pascal, and has 4.6K bytes of data space (constants,
error messages, etc.) a copy of which is allocated to each executing process.
.NH 2
Format of the object file
.PP
.I Px
normally interprets the code left in an object file by a run of the
Pascal translator
.I pi.
The file where the translator puts the object originally, and the most
commonly interpreted file, is called
.I obj.
In order that all persons using
.I px
share a common text image, this executable file is 
a small process that coordinates with the interpreter to start
execution.
The interpreter code is placed
at the end of a special ``header'' file and the size of the initialized
data area of this header file is expanded to include this code,
so that during execution it is located at an
easily determined address in its data space.
When executed, the object process creates a
.I pipe ,
creates another process by doing a
.I fork ,
and arranges that the resulting parent process becomes an instance of
.I px .
The child process then writes the interpreter code through 
the pipe that it has to the
interpreter process parent.
When this process is complete, the child exits.
.PP
The real advantage of this approach is that it does not require modifications
to the shell, and that the resultant objects are ``true objects'' not
requiring special treatment.
A simpler mechanism would be to determine the name of the file that was
executed and pass this to the interpreter.
However it is not possible to determine this name
in all cases.\*(Dd
.FS
\*(dd\ For instance, if the
.I pxref
program is placed in the directory
`/usr/bin'
then when the user types
``pxref program.p''
the first argument to the program, nominally the programs name, is
``pxref.''
While it would be possible to search in the standard place,
i.e. the current directory, and the system directories
`/bin'
and
`/usr/bin'
for a corresponding object file,
this would be expensive and not guaranteed to succeed.
Several shells exist that allow other directories to be searched
for commands, and there is,
in general,
no way to determine what these directories are.
.FE
.NH 2
General features of object code
.PP
Pascal object code is relocatable as all addressing references for
control transfers within the code are relative.
The code consists of instructions interspersed with inline data.
All instructions have a length that is an even number of bytes.
No variables are kept in the object code area.
.PP
The first byte of a Pascal interpreter instruction contains an operation
code.
This allows a total of 256 major operation codes, and 232 of these are
in use in the current
.I px.
The second byte of each interpreter instruction is called the
``sub-operation code'',
or more commonly the
.I sub-opcode.
It contains a small integer that may, for example, be used as a
block-structure level for the associated operation.
If the instruction can take a longword constant,
this constant is often packed into the sub-opcode
if it fits into 8 bits and is not zero.
A sub-opcode value of zero specifies that the constant would not
fit and therefore follows in the next word.
This is a space optimization, the value of zero for flagging
the longer case being convenient because it is easy to test.
.PP
Other instruction formats are used.
The branching
instructions take an offset in the following word,
operators that load constants onto the stack 
take arbitrarily long inline constant values,
and many operations deal exclusively with data on the
interpreter stack, requiring no inline data.
.NH 2
Stack structure of the interpreter
.PP
The interpreter emulates a stack-structured Pascal machine.
The ``load'' instructions put values onto the stack, where all
arithmetic operations take place.
The ``store'' instructions take values off the stack
and place them in an address that is also contained on the stack.
The only way to move data or to compute in the machine is with the stack.
.PP
To make the interpreter operations more powerful
and to thereby increase the interpreter speed,
the arithmetic operations in the interpreter are ``typed''.
That is, length conversion of arithmetic values occurs when they are
used in an operation.
This eliminates interpreter cycles for length conversion
and the associated overhead.
For example, when adding an integer that fits in one byte to one that
requires four bytes to store, no ``conversion'' operators are required.
The one byte integer is loaded onto the stack, followed by the four
byte integer, and then an adding operator is used that has, implicit
in its definition, the sizes of the arguments.
.NH 2
Data types in the interpreter
.PP
The interpreter deals with several different fundamental data types.
In the memory of the machine, 1, 2, and 4 byte integers are supported,
with only 2 and 4 byte integers being present on the stack.
The interpreter always converts to 4 byte integers when there is a possibility
of overflowing the shorter formats.
This corresponds to the Pascal language definition of overflow in
arithmetic operations that requires that the result be correct
if all partial values lie within the bounds of the base integer type:
4 byte integer values.
.PP
Character constants are treated similarly to 1 byte integers for
most purposes, as are Boolean values.
All enumerated types are treated as integer values of
an appropriate length, usually 1 byte.
The interpreter also has real numbers, occupying 8 bytes of storage,
and sets and strings of varying length.
The appropriate operations are included for each data type, such as
set union and intersection and an operation to write a string.
.PP
No special
.B packed
data formats are supported by the interpreter.
The smallest unit of storage occupied by any variable is one byte.
The built-ins
.I pack
and
.I unpack
thus degenerate to simple memory to memory transfers with
no special processing.
.NH 2
Runtime environment
.PP
The interpreter runtime environment uses a stack data area and a heap
data area, that are kept at opposite ends of memory
and grow towards each other.
All global variables and variables local to procedures and functions
are kept in the stack area.
Dynamically allocated variables and buffers for input/output are
allocated in the heap.
.PP
The addressing of block structured variables is done by using
a fixed display
that contains the address of its stack frame
for each statically active block.\*(Dg
.FS
\*(dg\ Here ``block'' is being used to mean any
.I procedure ,
.I function
or the main program.
.FE
This display is referenced by instructions that load and store
variables and maintained by the operations for
block entry and exit, and for non-local
.B goto
statements.
.NH 2
Dp, lc, loop
.PP
Three ``global'' variables in the interpreter, in addition to the
``display'', are the
.I dp,
.I lc,
and the
.I loop.
The
.I dp
is a pointer to the display entry for the current block;
the
.I lc
is the abstract machine location counter;
and the
.I loop
is a register that holds the address of the main interpreter
loop so that returning to the loop to fetch the next instruction is
a fast operation.
.NH 2
The stack frame structure
.PP
Each active block
has a stack frame consisting of three parts:
a block mark, local variables, and temporary storage for partially
evaluated expressions.
The stack in the interpreter grows from the high addresses in memory
to the low addresses,
so that those parts of the stack frame that are ``on the top''
of the stack have the most negative offsets from the display
entry for the block.
The major parts of the stack frame are represented in Figure 1.1.
.so fig1.1.n
Note that the local variables of each block
have negative offsets from the corresponding display entry,
the ``first'' local variable having offset `\-2'.
.NH 2
The block mark
.PP
The block mark contains the saved information necessary
to restore the environment when the current block exits.
It consists of two parts.
The first and top-most part is saved by the
.SM CALL
instruction in the interpreter.
This information is not present for the main program
as it is never ``called''.
The second part of the block mark is created by the
.SM BEG
begin block operator that also allocates and clears the
local variable storage.
The format of these blocks is represented in Figure 1.2.
.sp
.so fig1.2.n
.PP
The data saved by the
.SM CALL
operator includes the line number
.I lino
of the point of call,
that is printed if the program execution ends abnormally;
the location counter
.I lc
giving the return address;
and the current display entry address
.I dp
at the time of call.
.PP
The
.SM BEG
begin operator saves the previous display contents at the level
of this block, so that the display can be restored on block exit.
A pointer to the beginning line number and the
name of this block is also saved.
This information is stored in the interpreter object code in-line after the
.SM BEG
operator.
It is used in printing a post-mortem backtrace.
The saved file name and buffer reference are necessary because of
the input/output structure
(this is discussed in detail in 
sections 3.3 and 3.4).
The top of stack reference gives the value the stack pointer should
have when there are no expression temporaries on the stack.
It is used for a consistency check in the
.SM LINO
line number operators in the interpreter, that occurs before
each statement executed.
This helps to catch bugs in the interpreter, that often manifest
themselves by leaving the stack non-empty between statements.
.PP
Note that there is no explicit static link here.
Thus to set up the display correctly after a non-local
.B goto
statement one must ``unwind''
through all the block marks on the stack to rebuild the display.
.NH 2
Arguments and return values
.PP
A function returns its value into a space reserved by the calling
block.
Arguments to a
.B function
are placed on top of this return area.
For both
.B procedure
and
.B function
calls, arguments are placed at the end of the expression evaluation area
of the caller.
When a
.B function
completes, expression evaluation can continue
after popping the arguments to the
.B function
off the stack,
exactly as if the function value had been ``loaded''.
The arguments to a
.B procedure
are also popped off the stack by the caller
after its execution ends.
.KS
.PP
As a simple example consider the following stack structure
for a call to a function
.I f,
of the form ``f(a)''.
.so fig1.3.n
.KE
.PP
If we suppose that
.I f
returns a
.I real
and that
.I a
is an integer,
the calling sequence for this function would be:
.DS
.TS
lp-2w(8) l.
PUSH    \-8
RV4:\fIl        a\fR
CALL:\fIl       f\fR
POP     4
.TE
.DE
.ZP
Here we use the operator
.SM PUSH
to clear space for the return value,
load
.I a
on the stack with a ``right value'' operator,
call the function,
pop off the argument
.I a ,
and can then complete evaluation of the containing expression.
The operations used here will be explained in section 2.
.PP
If the function
.I f
were given by
.LS
 10 \*bfunction\fR f(i: integer): real;
 11 \*bbegin\fR
 12     f := i
 13 \*bend\fR;
.LE
then
.I f
would have code sequence:
.DS
.TS
lp-2w(8) l.
BEG:2   0
        11
        "f"
LV:\fIl\fR      40
RV4:\fIl\fR     32
AS48
END
.TE
.DE
.ZP
Here the
.SM BEG
operator takes 9 bytes of inline data.
The first byte specifies the 
length of the function name.
The second longword specifies the
amount of local variable storage, here none.
The succeeding two lines give the line number of the
.B begin
and the name of the block
for error traceback.
The
.SM BEG
operator places a name pointer in the block mark.
The body of the
.B function
first takes an address of the
.B function
result variable
.I f
using the address of operator
.SM LV
.I a .
The next operation in the interpretation of this function is the loading
of the value of
.I i .
.I I
is at the level of the
.B function
.I f ,
here symbolically
.I l,
and the first variable in the local variable area.
The
.B function
completes by assigning the 4 byte integer on the stack to the 8 byte
return location, hence the
.SM AS48
assignment operator, and then uses the
.SM END
operator to exit the current block.
.NH 2
The main interpreter loop
.PP
The main interpreter loop is simply:
.DS
.mD
iloop:
        \fBcaseb\fR     (lc)+,$0,$255
        <table of opcode interpreter addresses>
.DE
.ZP
The main opcode is extracted from the first byte of the instruction
and used to index into the table of opcode interpreter addresses.
Control is then transferred to the specified location.
The sub-opcode may be used to index the display,
as a small constant,
or to specify one of several relational operators.
In the cases where a constant is needed, but it
is not small enough to fit in the byte sub-operator,
a zero is placed there and the constant follows in the next word.
Zero is easily tested for,
as the instruction that fetches the
sub-opcode sets the condition code flags.
A construction like:
.DS
.mD
_OPER:
        \fBcvtbl\fR     (lc)+,r0
        \fBbneq\fR      L1
        \fBcvtwl\fR     (lc)+,r0
L1:     ...
.DE
is all that is needed to effect this packing of data.
This technique saves space in the Pascal
.I obj
object code.
.PP
The address of the instruction at
.I iloop
is always contained in the register variable 
.I loop .
Thus a return to the main interpreter is simply:
.DS
        \fBjmp\fR       (loop)
.DE
that is both quick and occupies little space.
.NH 2
Errors
.PP
Errors during interpretation fall into three classes:
.DS
1) Interpreter detected errors.
2) Hardware detected errors.
3) External events.
.DE
.PP
Interpreter detected errors include I/O errors and
built-in function errors.  
These errors cause a subroutine call to an error routine
with a single parameter indicating the cause of the error.
Hardware errors such as range errors and overflows are
fielded by a special routine that determines the opcode
that caused the error.
It then calls the error routine with an appropriate error
parameter.
External events include interrupts and system limits such
as available memory.
They generate a call to the error routine with an 
appropriate error code.
The error routine processes the error condition, 
printing an appropriate error message and usually
a backtrace from the point of the error.