Apple Orchard 1980 Fall v1n2 reduced

PAGE 38
THE APPLE ORCHARD
FALL 1980
PASCAL OPERAND FORMATS
or, The Secret Life of a Variable
(or, Everything You Wanted to Know
About Pascal Variables, But Couldn't Get
Through To The Hotline To Ask)
by
Jo Kellner
The Apple _hotline has received
numerous calls about the internal
structure of Pascal variables. This
information can be very useful
when sending data (especially
complex data formats such as
strings) to an assembly routine from
a Pascal host program. This article
describes a few of the more
commonly used variable types. For
a complete description of the more
complex variables, including
records and arrays, see pages 202
through 204 of the Apple Pascal
reference manual.
Machine language (assembly)
routines are commonly used
when speed is critical, and when
the code must access other
assembly routines such as PROMs
or 1/0 drivers which can't be reassembled
as part of the program.
Also, most single-bit operations are
much easier to do in assembly than
in Pascal.
In the USCD Pascal system, it's a
fairly simple matter to create short
assembly programs which can be
linked into a Pascal host program.
In some cases, it may be sufficient
to merely call the assembly routine;
however, most routines require
data in order to be useful. The
means by which data is passed to or
from these routines is called a "parameter/}.
decimal 65535
A parameter. is a temporary
variable created by Pascal for the
purpose of passing data to or from a
subroutine. The term "formal
parameter" implies that the
address of the actual variable is
passed to the subroutine as a
parameter instead of its value.
Certain types of variables may be
passed by value, but any variable
may be passed by name by simply
declaring it to be a formal
parameter (a VAR). Pascal does not
allow parameters of variable length
(with the exception of certain sets
and long parameters) to be passed
on the CPU stack, since this could
exceed the stack capacity and crash
the operating system, so these
parameters are automatically used
as formal parameters. A good
explanation of the various ways of
passing parameters may be found
in Peter Grogono's book, " Programming
in Pascal/}.
Before delving into the details,
let's define some terms and
conventions which we'll use later
on:
BIT= a binary digit (0or1). A bit is
the smallest unit of information
which can be stored in a
computer.
NYBBLE = 4 bits (half a byte). A hexadecimal
digit is one nybble
(pronounced "nibble/}).
hexadecimal FFFF < ....... memory ....-> 0000 addresses
MSB
LSB
FIGURE 1
< ........ byte ... >
15 14 . . . 8 7 . . . . . 0 32767). By subtracting this
positive value from 65536, the
negative integer is revealed.
Similarly, large positive integers are
stored as a complementary
negative numbers (remember
Integer Basic). The sign bit (MSB) is
0 if positive, 1 if negative.
(see Figure 2)
Example: the number 3 is represented
in binary as:
MSB
LSB
00000000 00000011
However, -3 shows up as
MSB
LSB
11111111 11111101
which also reads as 65533 (or
65536-3)!
Integers may be passcu by value
or as formal parameters.
REALS
Real numbers, in USCD Pascal,
are floating point numbers in the
range of +/ -1 .17550E-38 to +/ -3.402
82E+38. Real numbers use four
bytes (2 words). The binary representation
is similar to the proposed
IEEE standard for floating numbers:
(See Figure 3, nex t page)