SIC/XE Architecture - GeeksforGeeks

20 May, 2019
SIC/XE stands for Simplified Instructional Computer Extra Equipment or Extra Expensive. This computer is an advance version of SIC. Both SIC and SIC/XE are closely related to each other that’s why they are Upward Compatible.
SIC/XE machine architecture:
1. Memory:
Memory consists of 8 bit-bytes and the memory size is 1 megabytes (220 bytes). Standard SIC memory size is very small. This change in the memory size leads to change in the instruction formats as well as addressing modes. 3 consecutive bytes form a word (24 bits) in SIC/XE architecture.
All address are byte addresses and words are addressed by the location of their lowest numbered byte.
2. Registers:It contain 9 registers (5 SIC registers + 4 additional registers). Four additional registers are:

        Mnemonics             Use of Register
           B                  Base register
           S                  General working register
           T                  General working register
           F                  Floating-point accumulator

3. Data Formats:
Integers are represented by Binary numbers.
Characters are represented using ASCII codes.
Floating points are represented using 48-bits.
4. Instruction formats:
In SIC/XE architecture there are 4 types of formats available
The Bit(e) is used to distinguish between Formats 3 and Formats 4,e=0 means Format 3 and e=1 means Format 4
e=0 means Format 3 and e=1 means Format 4
Format 1(1 byte):
Format 2(2 bytes):
Format 3(3 bytes):
Format 4(4 bytes):
n=Indirect mode, i=Immediate addressing, x=Index addressing, b=Base addressing, p= Program counter, e=Exponential addressing
5. Addressing Modes:To use Format 3 use of Base register and Program counter is there.
   
           Mode               Indication           Target Address (TA) 
           Base relative      b=1, p=0             TA=(B) + displacement 
           Program-counter    b=0, p=1             TA=(PC) + displacement 
           relative   

Target address is the effective address of the instruction.
6. Instruction Set:
IN SIC/XE all the instructions are same as that of SIC architecture but because of Floating point data format it provide Floating point Arithmetic functions too.
To perform floating-point arithmetic operations,ADDF = Add floating points, 
SUBF = Subtract floating points, 
MULF = Multiply floating points, 
DIVF = Divide floating points 
ADDF = Add floating points, 
SUBF = Subtract floating points, 
MULF = Multiply floating points, 
DIVF = Divide floating points 
SVC (Supervisor call) is also provided in the SIC/XE architecture to handle Interrupts.
7. Input and Output:SIC/XE architecture include I/O channels that allow to perform I/O operations while CPU is executing other tasks. It will allow overlapping of computing and I/O, which make this architecture more efficient. Instructions such as SIO, TIO, HIO are used to start, test, and halt the operation I/O channels.
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