Slide S1-01


Chapter 1  -  BACKGROUND


Simplified Instructional Computer (SIC)


Memory

   . 8-bit bytes

   . 3 consecutive bytes = one word (24 bits)

   . all addresses are byte addresses

   . words are addressed by the location of their lowest
     numbered byte
                      15
   . total of 32768 (2  ) bytes



Register 

   . 5 registers

   . all have special uses

   . each 24 bits

       Mnemonics |  Number  |  Special use
     ------------+----------+-----------------------
          A      |    0     |  Accumulator
          X      |    1     |  Index register
          L      |    2     |  Linkage register
          PC     |    8     |  Program Counter
          SW     |    9     |  Status Word (contains
                 |          |  Condition Code (CC))





                                                           Slide S1-02


Data Formats

   . integers
        24-bit binary number
        2's complement for negative

   . characters
        8-bit ASCII code

   . no floating point


Instruction Format


               8       1       15
           ----------------------------
          |  opcode  | x |   address   |
           ----------------------------
                        \
                         indicates indexed addressing


Addressing Modes

    Mode    |  Indication   | Target address calculation
   ---------+---------------+----------------------------
    Direct  |    x = 0      |  TA = address
    Indexed |    x = 1      |  TA = address + (X)
            |               |





                                                           Slide S1-03


Instruction Set

   . load and store registers

   . integer arithmetic
        involves register A and a word in memory;
        result in register A
      
   . comparison
        register A with a word;
        set condition code (CC)   
   
   . conditional jump

   . subroutine linkage
        jump to subroutine
        return from subroutine


Input and Output

   . transfer one byte to or from the rightmost 8 bits of
     register A

   . Test Device
        sets CC

   . Read Data

   . Write Data


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                                                           Slide S1-04


SIC/XE


Memory
                    20
   . one megabyte (2  )


Registers

   . additional registers

      
       Mnemonics |  Number  |  Special use
     ------------+----------+------------------------------
          B      |    3     |  Base register
          S      |    4     |  General working register
          T      |    5     |  General working register
          F      |    6     |  Floating-point accumulator
                 |          |


Data Format

   . 48-bit floating-point data type

          1     11        36
         --------------------------
        | s |  exp  |   fraction   |
         --------------------------
               /            \
            0 - 2047       0 - 1
              e              f
         - - - - - - - - - - - - - - 
                   (e - 1024)
              f * 2





                                                           Slide S1-05


Instruction Format

   . an address (in general) will not fit into a 15-bit field

   . Format 1 (1 byte):

              8
           ------
          |  op  |
           ------

   . Format 2 (2 bytes):

              8    4    4
           ----------------
          |  op  | r1 | r2 |
           ----------------

   . Format 3 (3 bytes):

              6    1   1   1   1   1   1      12
           -----------------------------------------
          |  op  | n | i | x | b | p | e |   disp   |
           -----------------------------------------

   . Format 4 (4 bytes):
                                                                         
              6    1   1   1   1   1   1       20
           --------------------------------------------
          |  op  | n | i | x | b | p | e |   address   |
           --------------------------------------------
                                        \
                                         Format 3 or 4


Addressing Modes

   . base relative            \          /  1 0
   . program-counter relative  >  b, p  <   0 1
   . direct                   /          \  0 0

   . indexed  }  x

   . immediate  \          /  0 1
   . indirect    >  n, i  <   1 0
   . simple     /          \  1 1 (0 0)





                                                           Slide S1-06


Instruction Set

   . load and store new registers

   . floating-point arithmetic operations

   . instructions that take their operands from registers
        register move
        register-register arithmetic operations

   . supervisor call


Input and Output

   . there are I/O channels

   . Start I/O channel

   . Test I/O channel

   . Halt I/O channel


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