Computer Architecture and Organization: L07: Control Sequence By: A. H. Abdul Hafez Abdul.hafez@hku.edu.tr, ah.abdulhafez@gmail.com CAO, by Dr. A.H. Abdul Hafez, CE Dept. HKU January 18, 2019

Outlines Review of instruction set. Timing and control Control unit Hardwired control Programmed control Control unit Instruction decoder Control sequencer Time diagram End CAO, by Dr. A.H. Abdul Hafez, CE Dept. HKU January 18, 2019

Memory Reference Instructions SYMBOL HEX CODE DESCRIPTION I=0 I=1 AND 0xxx 8xxx AND memory word to AC ADD 1xxx 9xxx Add memory word to AC LDA 2xxx Axxx Load memory word to AC STA 3xxx Bxxx Store Content of AC in Memory BUN 4xxx Cxxx Branch unconditionally BSA 5xxx Dxxx Branch and Save return address ISZ 6xxx Exxx Increment and Skip if zero Where xxx represents the memory address in hexa.

Register Reference Instructions SYMBOL HEX DESCRIPTION CLA 7800 Clear AC CLE 7400 Clear E CMA 7200 Complement AC CME 7100 Complement E CIR 7080 Circulate right AC and E CIL 7040 Circulate left AC and E INC 7020 Increment AC SPA 7010 Skip next instruction if AC positive SNA 7008 Skip next instruction if AC negative SZA 7004 Skip next instruction if AC zero SZE 7002 Skip nexyt instruction if E is 0 HLT 7001 Halt computer register reference instructions are performed in a single clock cycle.

I/O Instructions SYMBOL HEX DESCRIPTION INP F800 Input Character to AC OUT F400 Output Character from AC SKI F200 Skip on Input Flag SKO F100 Skip on Output Flag ION F080 Interrupt on IOF F040 Interrupt off I/O instructions are performed in a single clock cycle.

Timing and control The timing for all registers in the basic computer is controlled by a master clock generator The clock pulses do not change the state of a register unles the register is enabled by a control signal ( LD, INR or CLR) The control signals are generated in the control unit and provide control inputs for the: multiplexers in the common bus, control inputs in processor registers, and microoperations for the accumulator Control sequence is generated in hardwired control and programmed control. CAO, by Dr. A.H. Abdul Hafez, CE Dept. HKU January 18, 2019

Hardwired Control The control logic is implemented with gates, flip flops, decoders, and other digital circuits. It has the advantage that it can be optimized to produce a fast mode of operation. Requires changes in the wiring among the various components if the design has to be modified or changed CAO, by Dr. A.H. Abdul Hafez, CE Dept. HKU January 18, 2019

Microprogrammed Control The control information is stored in a control memory. The control memory is programmed to initiate the required sequence of microoperations. Any required changes or modifications can be done by updating the microprogram in control memory CAO, by Dr. A.H. Abdul Hafez, CE Dept. HKU January 18, 2019

Control Unit of “The Basic Computer” 15 14 13 12 11-0 3x8 decoder I Control logic gates 7 6 5 4 3 2 1 0 D0 D7 ... T15 4x6 15 14 .... 2 1 0 . . . 4-bit Sequence Counter (SC) Increment (INR) Clear (CLR) Clock T0 instruction register (IR) CAO, by Dr. A.H. Abdul Hafez, CE Dept. HKU January 18, 2019

Sequence Counter 4x6 decoder 15 14 .... 2 1 0 . . . 4-bit Sequence Counter (SC) Increment (INR) Clear (CLR) Clock T0 ... As an example, consider the case where SC is incremented to provide timing signals T0,T1,T2,T3 and T4 in sequence. At time T4, SC is cleared to 0 if decoder output D3 is active D3T4: SC  0 CAO, by Dr. A.H. Abdul Hafez, CE Dept. HKU January 18, 2019

Timing Diagram of Sequence Counter Example: T0, T1, T2, T3, T4, T0, T1, . . . Assume: At time T4, SC is cleared to 0 if decoder output D3 is active D3T4: SC  0 CAO, by Dr. A.H. Abdul Hafez, CE Dept. HKU January 18, 2019

Why use a sequence counter? Not all instructions are executed in one clock cycle! This means, one instruction (an opcode) may consist of several microoperations. Using timing steps and microoperations, we can describe the instruction fetch and execution operations. First to do is: T0 : AR  PC CAO, by Dr. A.H. Abdul Hafez, CE Dept. HKU January 18, 2019

The end of the Lecture Thanks for your time Questions are welcome CAO, by Dr. A.H. Abdul Hafez, CE Dept. HKU January 18, 2019