COMPUTER INSTRUCTIONS & TIMING & CONTROL.
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The document outlines the instruction code formats of a basic computer, which include memory-reference, register-reference, and input-output instructions. It explains the bit allocation for addressing modes, operation codes, and the essential differences among these instruction types. Additionally, it covers clock pulses and control organization methods, including hardwired control and micro-program control.
COMPUTER INSTRUCTIONS & TIMING & CONTROL.
- 2. The basic computer has three instruction code formats which are :- MEMORY- REFERENCE INSTRUCTION A memory – reference instruction uses 12 bits to specify an address and 1 bit to specify the addressing mode I. I is equal to 0 for direct address and to 1 for indirect address REGISTER- REFERENCE INSTRUCTION A register reference instruction specifies an operation on or a test of the AC register .An operand from the memory is not needed therefore the other 12 bits are used to specify the operation to be executed. INPUT-OUTPUT INSTRUCTION An input-output instruction does not need a reference to memory and is recognized by operational code 111 with a 1 in the left most bit of the instruction the remaining 12 bits are used to specify the type of input output operation or test performed.
- 3. Basic Computer Instruction Format 15 14 12 11 0 I Opcode Address Memory-Reference Instructions (OP-code = 000 ~ 110) Register-Reference Instructions (OP-code = 111, I = 0) Input-Output Instructions (OP-code =111, I = 1) 15 12 11 0 Register operation0 1 1 1 15 12 11 0 I/O operation1 1 1 1
- 4. 3 Instruction Code Formats : Memory-reference instruction Register-reference instruction Input-Output instruction I Opcode Address 15 14 12 11 0 I=0 : Direct, I=1 : Indirect 0 1 1 1 Register Operation 15 14 12 11 0 1 1 1 1 I/O Operation 15 14 12 11 0 Hex Code Symbol I = 0 I = 1 Description 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 CLA 7800 Clear AC CLE 7400 Clear E CMS 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 next instruction if E is 0 HLT 7001 Halt computer 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
- 5. The timing for all register in basic computer is controlled by a master clock generator. Clock pulses – The clock pulses are applied to all flip-flops and registers in the system , including the flip-flops in the control unit . The clock pulses do not change the state of a register unless the register is unable by a control signal. There are two types of control organizations :- 1. Hardwired control – In hardwired organization, 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. 2. Micro program control – The control information is stored in the control memory . The control memory is programmed to initiate the required sequence of micro operations.
- 6. Control unit of Basic Computer Instruction register (IR) 15 14 13 12 11 - 0 3 x 8 decoder 7 6 5 4 3 2 1 0 I D0 15 14 . . . . 2 1 0 4 x 16 decoder 4-bit sequence counter (SC) Increment (INR) Clear (CLR) Clock Other inputs Control signals D T T 7 15 0 Combinational Control logic