SIC ARCHTECTURE


  SIC is a hypothetical computer that has been carefully designed to include the hardware features most often found on real machines, while avoiding unusual or irrelative complexities.    SIC Machine Architecture
           ************************
1)MEMORY :
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– Memory consists of 8-bit bytes.
– Any three consecutive bytes form a word.
– There are 32,768 (2^15) bytes in the computer memory.

2)REGISTERS :
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– There are five registers all of which have special uses.
– Each register is 24 bits in length.

   Mnemonic    Special use
   ********    ***********
      A        Accumulator; used for arithmetic operations.
      X        Index register; used for addressing.
      L        Linkage register; The JSUB (Jump to subroutine) stores                  the return address in this register.
      PC       Program counter; contains the address of next                           instruction to be fetched for execution.
      SW       Status word; contains a variety of information ,                        including a condition code(CC).

3)DATA FORMATS:
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– Integers are stored as 24-bit binary numbers.
– 2’s complement method is used for negative values.
– Characters are stored using their 8-bit ASCII codes.
– There is no floating point hardware on SIC.

4)INSTRUCTION FORMATS:
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– All machine instructions on the standard version of SIC have the following 24-bit  format.

         8           1          15
    —————————————————
   |   opcode      | x|       address                  |
    —————————————————

– The flag bit ‘x’ is used to indicate indexed addressing mode.

5)ADDRESSING MODES:
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– The target address is calculated from the address given in the   instruction as follows.

   Mode             Indication           Target address calculation
   ****************************************************************
   Direct           x=0                  TA = address
   Indexed          x=1                  TA = address + (X)
   [   (X) represents the contents of register X  ]

6)INSTRUCTION SET:
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CLICK THIS LINK TO SEE THE COMPLETE LIST OF INSTRUCTION SET OF SIC

7)INPUT AND OUTPUT:
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– The following are previlized instructions that perform I/O i.e. they are under the control of OPERATING SYSTEM and not in controlled by user program.

– Test Device (TD) instruction sets the condition code (CC) to indicate the result of the test.A setting of < means the device is ready to send or receive a byte of data ,  and =  means the device is not ready.

– Read Device (RD) and Write Device (WR) transfer a byte of data from and to the device specified in the address field.

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