Register-reference instructions are recognized by the control when 07 = 1 and I = 0. These instructions use bits 0 through 11 of the instruction code to specify one of 12 instructions. These 12 bits are available in IR(0-11). They were also transferred to AR during time T2•
The control functions and microoperations for the register-reference instructions are. listed in Table 5-3. These instructions are executed with the clock transition associated with timing variable T3• Each control function needs the Boolean relation D7I'T3, which we designate for convenience by the symbol r. The control function is distinguished by one of the bits in IR(0-11). By assigning the symbol B, to bit i of IR, all control functions can be simply denoted by rB;. For example, the instruction CLA has the hexadecimal code 7800 (see Table 5-2), which gives the binary equivalent 011I 1000 0000 0000. The first bit is a zero and is equivalent to I'. The next three bits constitute the operation code and are recognized from decoder output D7• Bit 11 in IR is I and is recognized from 811• The control function that initiates the rnicrooperation for this instruction is D7I'T3B11 = rB11• The execution of a register-reference instruction is completed at time T3• The sequence counter SC is cleared to 0 and the control goes back to fetch the next instruction with timing signal T0•
The first seven register-reference instructions perform clear, complement, circular shift, and increment rnicrooperations on the AC or E registers. The next four instructions cause a skip of the next instruction in sequence when a stated condition is satisfied. The skipping of the instruction is achieved by incrementing PC once again (in addition, it is being incremented during the fetch phase at time T1). The condition control statements must be recognized as part of the control conditions . The AC is positive when the sign bit in AC(IS) = 0; it is negative when AC(IS) = I. The content of AC is zero (AC = 0) if all the flip-flops of the register are zero. The HLT instruction clears a start-stop flip-flop S and stops the sequence counter from counting. To restore the operation of the computer, the start-stop flip-flop must be set manually.