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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

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

Simplified Instructional Computer (SIC)

Instruction Formats:

x: flag bit used to indicate indexed-addressing mode

Simplified Instructional Computer (SIC)

Addressing Mode:

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

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)

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.

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

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

Instruction Formats

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)

Number of addresses is larger. (220 as compared to 215).

Some instructions do not require operands.

Format 1:

Format 2:

Format 3:

Format 4:

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).

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.

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).

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

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