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Illustrations of instruction word formats I, II, and III
Extended  Operand  Addressing

Fire Controlman Volume 03-Digital Data Systems
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Figure 8-6.—Illustration of instruction word format IV-A. word. First the computer executes the upper half-word instruction then the lower. If only one of these format instructions is to be stored in a memory word, then it is stored in the upper half-word location. An active status register  (ASR)  bit  (215)  is  used  to  keep  track  of upper/lower half instruction execution. Format IV-A instructions are used for a variety of computer operations that do not require an operand or operand address to be part of the instruction. These operations include but are not limited to mathematics and comparison operations, IOC commands, task and executive state operations, and real-time or monitor clock operations. The format IV-A instruction (fig. 8-6) is made up of an f field, a field, f4 field, an index designator (b) field, and i field, which is unused unless specified. The only field we have not covered is the f4 field, a 3-bit subfunction code. This field can be used to identify code memory registers (CMR) for CMR operations. Format IV-B instructions are used to shift data stored in an accumulator. The accumulator designator specifies an accumulator in control memory. The shift count designator specifies a shift count or a source of a shift count. Instruction format IV-B (fig. 8-7) is made up of an f field, an a field, and a shift designator (m) field. Format IV-C —Format IV-C instructions (fig. 8-8) are used for individual bit operations. These operations  include  setting,  clearing,  or  testing  an individual bit of a specified accumulator register. The 5-bit n field provides the bit position pointer to specify the register bit to be operated on. Format V —Format V instructions are full-word format  instructions  (fig.  8-9)  used  for  single  and double-precision floating-point math operations and other large magnitude number functions. In this format the f, f5, and f6 fields are used to define the specific operation to take place. The a and b fields are used for accumulator and index register definition. The m field provides decimal point positioning values for floating point operations. INSTRUCTION OPERAND ADDRESSING The types of operand addressing usually available are direct, extended, immediate, implicit, indexed, indirect, and relative. Direct Operand Addressing In direct operand addressing, the address of the operand’s   memory   location   is   contained   in   the instruction. Figure 8-10 shows an example of direct addressing format. Figure 8-7.—Illustration of instruction word format IV-B. 8-10 Figure 8-8.—Il1ustration of instruction word format IV-C.






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