STA: Store AC & BUN: Branch Unconditionally
STA: Store AC
This instruction stores the content of AC into the memory word specified by the effective address. Since the output of AC is applied to the bus and the data input of memory is connected to the bus, we can execute this instruction with one microoperation:
D3T4: M [AR] <-- AC, SC <-- 0
BUN: Branch Unconditionally
This instruction transfers the program to the instruction specified by the effective address. Remember that PC holds the address of the instruction to be read from memory in the next instruction cycle. PC is incremented at time T1 to prepare it for the address of the next instruction in the program sequence. The BUN instruction allows the programmer to specify an instruction out of sequence and we say that the program branches (or jumps) unconditionally. The instruction is executed with one rnicrooperation:
D4T4: PC <-- AR, SC <-- 0
The effective address from AR is transferred through the common bus to PC . Resetting SC to 0 transfers control to T0• The next instruction is then fetched and executed from the memory address given by the new value in PC.