INSTRUCTION FORMAT
An instruction is a coded statem.ent which orders the com.puter to perform. a fundam.ental operation. A set of instructions suitably com.bined to perform. a specific task is called a program.
or routine.
As will be shown in Section V, the task of coding the instructions in a program. is greatly sim.plified by the use of the Easycoder sym.bolic program.m.ing system.. The Easycoder Assem.bly Program. converts the sym.bolic coding written by the programmer into a m.achine language which is acceptable to the internal logic of the machine.
OPERATION CODE
Basic to all instructions is an operation code, usually referred to as an op code, that de-fines the fundam.ental operation to be perform.ed. The program.m.er specifies an op code by using a predefined m.nem.onic configuration; e. g., BA is the op code that specifies a "binary add"
operation, MCW is the op code that specifies a "m.ove characters to word m.ark" operation. The Easycoder Assem.bly Program. autom.atically converts a m.nem.onic op code into a single-charac-ter, m.achine-Ianguage op code and sets the word-mark bit in the character position in which it is stored.
A AND B ADDRESSES
Most instructions also have two address portions, designated as the A address and the B address. The address portions indicate the starting locations of the operand fields in the m.ain m.em.ory. Using the Easycoder language, the program.m.er can specify m.em.ory locations by m.eans of syrn.bolic addresses or "tags" (see Section V).
The Easycoder Assem.bly Program. autom.atically assigns absolute m.em.ory addresses to the syrn.bolic addresses appearing in a program. (see Figure 3-1). Thus, the program.m.er can m.anipulate operands without regard to their actual storage locations in m.em.ory.
SYMBOLIC ADDR.
(TAG)
ABSOLUTE MEMORY ADDRESS
Figure 3 -1. Conversion of Sym.bolic Tags to Absolute Mem.ory Addresses
Because of the modular design of Series 200 computers, the programmer has the facility to specify whether a two-, three-, or four-character absolute address will be assigned to each
~ymbolic address used in the program. In any case, the absolute addresses assigned by the assembly program are interpreted as pure binary numbers (see Section IV).
VARIANT CHARACTER
The variant character is used to modify the op code of an instruction. For example, the op code of a Branch on Condition Test instruction (BeT) specifies the fundamental operation
"branch if a tested condition is met." The condition or restriction which must be met before the branch can occur is specified by the variant character. A table of valid variant characters is presented in Appendix B.
SUMMARY
Figure 3-2 shows the six basic formats in which machine-language instructions may appear.
Since the maximum number of characters in an instruction depends upon whether two-, three-, or four-character addressing is being used, shaded boxes in the illustration indicate the format of an instruction without specifying the number of characters in each part. These formats are representative of all instructions except those associated with input/output and translate opera-tions. 1
For the sake of direct comparisons, Figure 3-3 illustrates each of the formats defined in Figure 3-2 as a symbolic entry on the programmer's coding form~
1
OP CODE
I
A ADDRESSI I
B ADDRESSI
VARIANT CHARACTER(S)2
OP CODEI
A ADDRESSI I
B ADDRESsl3 OP CODE
I
A ADDRESSI
VARIANT CHARACTER(S)4
OP CODEI
A ADDRESSI
5 OP CODE VARIANT
CHARACTER(S)
6
OP CODEFigure 3-2. Series 200 Instruction Formats
The format of an input/output instruction is a modification of format 3 shown in Figure 3-2.
Specifically, the variant characters of the instruction are replaced by a field of one or more control characters which define the input/output operation in terms of data path, direction of data flow, control unit designation, etc. The format of a translate instruction is a modification of format 1 shown in Figure 3 -2. In Section VIII, Series 200 instructions are described in terms of their individual formats.
3-3 #2-139
SECTION III. DATA FORMAT
EASYCODER
CODING FORM
PROBLEM _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ PROGRAMMER _ _ _ _ _ _ DATE _ _ _ _ _ PAGE_OF_
CARD fl~ LOCATION OPERATION OPERANDS
NUMBER ~ ~ CODE
I 213 4 5 6 7 8 1415 2021 6263 80
I
seE p,
LA BEL .0.G, I FORMAT 1I
i
IA
II T E.,~.'- -rOT ~I FORMAT 2I I
,
I
i :
BeT ISZl<:o •. ~~ FORIMAT 3-.1I I
i SW
WORI( FORMAT 4i I ,. I
I 1 .,
'CA,h\ 6.G.
FORMAT 510 1
I I
I
I
~ FORMAT 612 I
I I I
13
14 1 I
15 I I
Figure 3-3. Symbolic Representation of Series 200 Instructions
ORGANIZATION OF DATA IN MAIN MEMORY
Data may be stored in the main memory in any of the following variable-length formats:
• FIELD
• ITEM
• RECORD
FIELDS
Consider the eight consecutive storage locations shown in Figure 3-4. To indicate to the machine that these eight characters are to be treated as a field, their left and right boundaries must be defined. The left boundary is normally defined by setting a word mark in position 990.
The right boundary is normally defined by specifying storage address 997 in the instruction that will manipulate the field. 1 The eight-character group shown in Figure 3-5 is thus defined as a field.
STORAGE ADDRESS CONTENTS
Figure 3-4. Consecutive Storage Locations ih Main Memory
1 Although this is the conventional method of defining fields, the Extended Move (EXM) instruc-tion (see Secinstruc-tion VIII) permits a field to be defined by a word mark at either the left or'the right boundary. The opposite boundary is then specified in the instruction.
PROBLEM
1. The leftmost character position can be defined in the instruction that will operate on the item and the rightmost character position defined by an item mark; or 2. The rightmost character position can be defined in the instruction that will
oper-ate on the item and the leftmost character position defined by an item mark.
NOTE: An item mark is illustrated in this manual by underlining the character with which it is associated. Fields within an item are defined by word marks.
There are only two instructions that manipulate items - Move Item and Translate, and Extended Move. In the Move Item and Translate instruction, the leftmost character of an item is addressed and the rightmost character contains an item mark. In the Extended Move instruc-tion, several different item boundaries can be specified by the variant character of the instruction.
Two items, each containing three data fields, are shown in Figure 3 -6.