SECTION S-) Computer Instructions 133 instruction is a register-reference type. If the bit is 1, the instruction is an input-output type. Note that the bit in position 15 of the instruction code is designated by the symbol I but is not used as a mode bit when the operation code is equal to 111. Only three bits of the instruction are used for the operation code. It may seem that the computer is restricted to a maximum of eight distinct operations. However, since register-reference and input-output instructions use the remain- ing 12 bits as part of the operation code, the total number of instructions can exceed eight. In fact, the total number of instructions chosen for the basic computer's equal to 25. . The instructions for the computer are listed in Table 5-2. The symbol designation is a three-letter word and represents an abbreviation intended for TABLE 5-2 Bask Computer Instructions Hexadecimal code Symbol 1-0 1-1 Out lux 200 AND ADD LDA STA BUN BSA ISZ Bux 9ut AXXX Bu xxx Dxxx Ex 4u SEX 6. A Mano machine code instruction has value OxB057. Use Table 5-2 and Figure 5-5 to answer the following: a. What is the 16-bit binary value of this instruction? b. What is the binary value of the opcode field? c. What is the binary value of the address field? d. What is the value of the Ibit? e. Suppose a label VAR1 is associated with the address in this instruction. What assembly command would generate the given machine code value? CLA CLE CMA CME CIR CIL INC SPA SNA SZA SZE HLT 7800 7400 7200 7100 7080 7040 7020 7010 7008 7004 7002 7001 Description AND memory word to AC Add memory word to AC Load memory word to AC Store content of AC in memory Branch unconditionally Branch and save return address Increment and skip if zero Clear AC Clear E Complement AC Complement E Circulate right AC and E Circulate left AC and E Increment AC Skip next instruction if AC positive Skip next instruction if AC negative Skip next instruction if AC zero Skip next instruction if Eis 0 Halt computer Input character to AC Output character from AC Skip on input flag Skip on output flag Interrupt on Interrupt off INP OUT SKI SKO ION F800 F400 F200 F100 F080 IOF F040