1. Lecture 1: SIC Architecture
COP 3402
Spring 2008

2. Simplified Instructional Computer (SIC)
Memory:
8-bit bytes
3 consecutive bytes form a word (24-bits)
Addresses are byte addresses
Words are addressed by location of their lowest numbered byte
Memory size = 32, 768 (2^15) bytes

3. Simplified Instructional Computer (SIC)
Registers:
Five registers
Registers are 24 bits of length

4. Simplified Instructional Computer (SIC)
Mnemonic
Number
Special Use A
Accumulator; used for arithmetic operations X 1
Index register; used for addressing L 2
Linkaged register; the Jump to Subroutine (JSUB) instruction stores the return address in this register PC 8
Program Counter; contains the address of the next instruction to be fetched for execution SW 9
Status word; contains a variety of information, including a Condition Code(CC)

5. Simplified Instructional Computer (SIC)
Data Formats:
Integers stored as 24-bit binary numbers
2’s complement representation is used for negative values
Characters stored using 8-bit ASCII codes

6. Simplified Instructional Computer (SIC)
Instruction Formats:
x: flag bit used to indicate indexed-addressing mode

7. Simplified Instructional Computer (SIC)
Addressing Mode:
Mode
Indication
Target Address Calculation
Direct
x = 0
TA=address
Indexed
x = 1
TA=address + (X)
Parenthesis are used to indicate the contents of a register or a memory location. (X) represents the content of register X

8. Simplified Instructional Computer (SIC)
Instruction Set:
Basic set of instructions, load and store registers (LDA, LDX, STA, STX,etc.), and arithmetic operations (ADD, SUB, MUL, DIV)
Arithmetic operations involve register A and a word in memory, and the result is left in the register.
Instruction COMP compares a value in A with a word in memory, and sets the condition code CC to indicate the result.
Jump instructions test the setting of CC
JSUB and RSUB instructions are used for subroutine linkage

9. Simplified Instructional Computer (SIC)
Input and Output:
Performed by transferring 1 byte at a time to or from the rightmost 8 bits of register A.
Each device is assigned a unique 8-bit code
Three I/O devices instructions:
Test device (TD): tests whether the addressed device is ready to send or receive a byte of data
Read data (RD)
Write data (WD)

10. SIC/XE SIC is upwards compatible with SIC/XE.
Memory arranged in bytes (Max = 220 bytes).
Can do floating point arithmetic.
Has more registers.
Has additional addressing modes.
Can do I/O in parallel with computation.

11. Registers Mnemonic Register Comment 3 B Base Register (for addressing)4 S
General Purpose Register 5 T 6 F
Floating point Accumalator (48-bits)

12. Data Formats
SIC/XE supports integers and characters in the same manner as SIC.
Introduces new 48-bit floating point type.
1-bit sign bit
11-bit exponent
36-bit fraction

13. Floating point format in SIC/XE
0 <= fraction <=1
Exponent is unsigned. Range = (0 – 2047)
Subtract 1024 from exponent to get correct value.
Value of any float will be.
Fraction * (2exponent-1024)
The sign of the number will be determined by the sign bit

14. Instruction Formats Format 1: Format 2: Format 3: Format 4:
Number of addresses is larger. (220 as compared to 215).
Some instructions do not require operands.
Format 1:
Format 2:
Format 3:
Format 4:
Op (1 byte)
Op (1 byte) R1 R2
Op (6 bit) n i x b p e
Disp (12 bit)
Op (6 bit) n i x b p e
Address(20 bit)

15. Formats (cont’d) n=1, i=0: The word at the target address is fetched.
n=0,i=1: The address is used as the operand.
n=i: value at address is taken as operand. (backward compatibility with SIC when used in Format 3)
x=0,1: enables/disables Indexed mode (as in SIC).

16. Formats (cont’d) b=1,p=0: Implies Base Relative Mode .
b=0,p=1: Implies Program Counter Relative Mode.
e determines whether mode 3 or mode 4 is in use.
e=0: Mode 3 is in use.
e=1: Mode 4 is in use.

17. Addressing Modes Base Relative: b=1, p=0
Target Address = B + disp
Program Counter Relative: b=0,p=1
Target Address = PC+disp
If b=p=0, then the address/disp field is taken as the address.
Indexed addressing may be used with both these modes (x=0,1).

18. Instructions Instructions to Load/Store new registers
LDB, STB,…
Floating point Arithmetic
ADDF, SUBF, MULF, DIVF
Register Instructions
ADDR, SUBR, MULR, DIVR, RMO
I/O instructions
TIO, SIO, HIO