IBM 709 General Information Manual page 15

know what is stored in location 0003. To assist the
programmer, the use of symbols can be extended to in-
clude addresses as well as the operation part of an in-
struction.
Assuming that the net pay for a payroll calculation
is stored in location 0003, the instruction might now
be written as "subtract net." This clarifies to the pro-
grammer the operation to be performed, and also what
quantity is involved. Thus, symbolic programming
employs instruction-by-instruction coding in a lan-
guage that is a representation of the basic language of
the computer itself.
The computer can execute instructions only if ,they
are stored in machine language. Thus, the instruction
"subtract net" must be converted to the basic machine
language before its execution. One way of doing this
would be to manually convert all symbolic instruc-
tions to machine language before entering them into
the computer. This obviously would be a laborious
task. A more practical solution is to have the com-
puter perform the conversion. This is accomplished
through use of a symbolic assembly program.
An assembly program performs the necessary trans-
lation or conversion from the symbolic language to
the machine language. At the same time, the program
assigns absolute core storage locations to the imtruc-
Convert the
Instructi on
"SUBTRACT NET"
Becomes
000100000010000000000000100000000
No
Yes
Print and punch the
resulting program
Figure 25. Flow Chart of an Assembly Program
14 IBM 709-7090
tions and data contained in the symbolic program.
Figure 25 shows a flow chart of the process involved
when using an assembly program. Assume that the
instructions to be assembled are on punched cards and
that the assembly program itself is on a magnetic tape
(assembly tape) .
The assembly program normally provides certain re-
strictions upon the length and type of symbols that
may be used by the programmer. It assigns an abso-
lute core storage location for the first symbolic instruc-
tion and increases this location by one for each sub-
sequent instruction decoded. In this manner, the de-
coded program instructions are assigned sequential lo-
cations. Space is provided for the data that are a part
of the assembled program. Normally, the constants
and other data are placed at the end of the symbolic
program and are assigned sequential locations that
follow the instruction area.
During assembly, the first reference to a symbolic
address assigns this address to an absolute storage lo-
cation. Any subsequent references to this symbolic ad-
dress will also be assigned the same address. The man-
ner in which the assembly proceeds is described below.
Each symbolic instruction is read from the card
reader into the central processing unit. The "sub-
tract" portion of the first instruction is matched
against the contents of the assembly tape and is de-
coded into the proper bit configuration for a subtract
operation 000100000010. The "net" portion of the
instruction is assigned a location in core storage that
is not being used. This bit configuration (assume
000 100000000) is then inserted into the address portion
of the decoded subtract instruction. The full instruc-
tion word (000100000010000000000000000 I 00000000),
both operation and address parts, is then written
as the first instruction on the program tape. The
original "subtract net" is stored so that it may appear
in the final printing of the program. The assembly
program then tests to determine if more instructions
remain to be decoded. If there are more, the above
process is repeated until all symbolic instructions have
been decoded and written on the program tape. When
the assembly process is complete, the resultant as-
sembled program is printed, together with the original
symbolic instructions. The print-out takes the follow-
ing form:
SYMBOLIC INSTRUCTION DECODED MACHINE EQUIVALENT
SUBTRACT NET
000100000010000000000000100000000
When the printing process is complete, the assembly
program furnishes the programmer with a group of