SIC ArchitectureAs given to GKM CSE-B students

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) Registers:

Five registers Registers are 24 bits of length

Simplified Instructional Computer (SIC)

Mnemonic Number Special Use

A 0 Accumulator; used for arithmetic operations

X 1 Index register; used for addressing

L 2 Linkage 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)

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

Addressing Mode:

Mode Indication Target Address Calculation

Direct x = 0 TA=address

Indexed x = 1 TA=address + (X)

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.

Registers

Mnemonic Register Comment

3 B Base Register (for addressing)

4 S General Purpose Register

5 T General Purpose Register

6 F Floating point Accumalator (48-bits)

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

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 bxi p e Disp (12 bit)

Op (6 bit) n bxi p e Address(20 bit)

Formats (cont’d)

n=1, i=0: The word at the target address is the address of the operand.

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