1.1 General Instruction Format:

All machine instructions on the standard version of SIC have the following 24-bit format:

The flag bit x is used to indicate indexed-addressing mode.

1.2 SIC/XE Instruction Format: "SIC == Simplified Instructional Computer"

The larger memory available on SIC/XE means that an address will (in general) no longer fit into a 15-bit field; thus the instruction format used on the standard version of SIC is no longer suitable.

There are two possible options - either use some form of relative addressing, or extend the address field to 20 bits. Both of these options are included in SIC/XE (Format 3 & 4).

In addition, SIC/XE provides some instructions that do not reference memory at all. Formats 1 and 2 in the following description are used for such instructions.

The new set of instruction formats is as follows:

Format-1:

8-bits - only opcode

Format-2:

16 bits - Register to Register operation

Format-3:

24 bits - (e=0) Relative Addressing

Format-4:

32 bits - (e=1) Extended Address

In-Detail:

Format 1 (1 byte):

contains only operation code (straight from table).

Format 2 (2 bytes):

first eight bits for operation code, next four for register 1 and following four for register 2. The numbers for the registers go according to the numbers indicated at the registers section (i.e, register T is replaced by hex 5, F is replaced by hex 6).

Format 3 (3 bytes):

First 6 bits contain operation code, next 6 bits contain flags, last 12 bits contain displacement for the address of the operand. Operation code uses only 6 bits, thus the second hex digit will be affected by the values of the first two flags (n and i). The flags, in order, are: n, i, x, b, p, and e. Its functionality is explained in the next section. The last flag e indicates the instruction format (0 for 3 and 1 for 4).

Format 4 (4 bytes):

same as format 3 with an extra 2 hex digits (8 bits) for addresses that require more than 12 bits to be represented.

The assembler goes through two passes translate the source program to object code.

Pass 1 - Definition pass

i. Assign addresses to all statement in the program

ii. Store values of all the labels in symbol table

iii. Do processing of declarative statements by assigning the memory

iv. Perform processing of assembler directives

Pass 2 - Translation pass

i. Assemble the instructions by converting mnemonics to op-code and operands to address

ii. Generate data values for constants

iii. Perform processing of assembler directives which is not done in pass 1

iv. Write the object code in appropriate file format

5.1 The Various Machine Independent assembler are as follows:

i. Literals

ii. Symbol-defining statements

iii. Expressions

iv. Program blocks

v. Control sections and program linking

5.2 Control Sections:

A

control section

is a part of the program that maintains its identity after assembly; each such control section can be loaded and relocated independently of the others.

Different

control sections

are most often used for subroutines or other logical subdivisions of a program. The programmer can assemble, load, and manipulate each of these control sections separately.

5.3

The Simple SIC Assembler

The assembler performs following functions to translate the

source program to object code

:

i. To convert mnemonics to op-code

ii. To convert symbolic operands to address

iii. To convert data constants to machine equivalent representation

iv. To resolve declarative statements and assembler directives

v. Write object code and assembly listing