Assembly Language Programming - Fundamentals of Computer Organization and Architecture - page 41

Operand
(Required in some
instructions)

Label
(Optional)

Operation Code
(Required)

Comment
(Optional)

Figure 3.2 Assembly instruction format

label differs from one assembly language to another. Some allow up to 32 characters
in length, others may be restricted to six characters. Assembly languages for some
processors require a colon after each label while others do not. For example, SPARC
assembly requires a colon after every label, but Motorola assembly does not. The
Intel assembly requires colons after code labels but not after data labels.
The operation code (opcode) field contains the symbolic abbreviation of a given
operation. The operand field consists of additional information or data that the
opcode requires. The operand field may be used to specify constant, label, immedi-
ate data, register, or an address. The comments field provides a space for documen-
tation to explain what has been done for the purpose of debugging and maintenance.
For the simple processor described in the previous section, we assume that the
label field, which may be empty, can be of up to six characters. There is no colon
requirement after each label. Comments will be preceded by “ ”. The simple

mnemonics of the ten binary instructions of Table 3.1 are summarized in Table 3.4.
Let us consider the following assembly instruction:

/

START LD X \ copy the contents of location X into AC

The label of the instruction LD X is START, which means that it is the memory
address of this instruction. That label can be used in a program as a reference as
shown in the following instruction:

BRA START \ go to the statement with label START

TABLE 3.4 Assembly Language for the Simple Processor

Mnemonic Operand Meaning of instruction

STOP Stop execution

LD x Load operand from memory (location x) into AC

ST x Store contents of AC in memory (location x)

MOVAC Copy the contents AC to DR

MOV Copy the contents of DR to AC

ADD Add DR to AC

SUB Subtract DR from AC

AND And bitwise DR to AC

NOT Complement contents of AC

BRA adr Jump to instruction with address adr

BZ adr Jump to instruction adr if AC ¼ 0