SIC/XE Instructions Encoding

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Questions and Answers

What is the format of the opcode for the LDA instruction in the given SIC/XE machine code?

000000 (correct)

000000000 (correct)

00000000 (correct)

What type of addressing mode is used in the instruction 'LDA 2 = 0010 x=0, b=0, p=1, e=0'?

Indexed

PC Relative (correct)

Direct

Base Relative

What is the purpose of the opcode in a machine code instruction?

Specifies the operation to be performed (correct)

Specifies the address of the operand

Specifies the type of instruction

Specifies the format of the instruction

What is the difference between the SIC and SIC/XE machine code?

SIC is an older version, while SIC/XE is an updated version Signup and view all the answers

What is the format of the operand in the instruction 'LDA 3600 = 0 011 0110 0000 0000'?

16-bit binary Signup and view all the answers

What type of assembler is needed to assemble the given SIC/XE machine code?

One-Pass Assembler Signup and view all the answers

What is the purpose of the EQU directive in assemblers?

Insert a name and value in SYMTAB Signup and view all the answers

In a one-pass assembler, how are forward references handled?

Forward references are not allowed Signup and view all the answers

Which assembler directive is used to assign values to names in a program?

ORG Signup and view all the answers

In the context of assemblers, what does the LTORG statement indicate?

Literals are defined and assigned memory addresses Signup and view all the answers

What is the purpose of LOCCTR in assemblers?

Tracking the location of instructions in the program Signup and view all the answers

How does a load-and-go assembler generate the object program?

By generating object code directly from the source on one pass Signup and view all the answers

What is the purpose of the START directive in Pass2: Assembling?

To specify the starting address of a program Signup and view all the answers

What is the difference between PC-relative and B-relative addressing?

PC-relative uses the program counter to calculate the address, while B-relative uses the base register Signup and view all the answers

What happens when a program is loaded at a specific address?

The program becomes absolute Signup and view all the answers

What is the purpose of the OPTAB table?

To store the opcode table Signup and view all the answers

What is the difference between a one-pass assembler and a multi-pass assembler?

One-pass assemblers scan the source code once, while multi-pass assemblers scan the source code multiple times Signup and view all the answers

What is the purpose of the symbol table (SYMTAB)?

To store the symbol definitions Signup and view all the answers

Study Notes

LDA instruction format

The LDA instruction uses a 3-byte format in the SIC/XE architecture:
- Byte 1: Opcode (3 bits)
- Byte 2: Address (15 bits)
- Byte 3: Address (15 bits)

Addressing Mode

The instruction 'LDA 2 = 0010' uses immediate addressing.
- Immediate addressing uses the actual value of the operand directly.
- The 'x=0, b=0, p=1, e=0' part represents the addressing mode in SIC/XE. In this case, p=1 indicates immediate addressing.

Purpose of Opcode

The opcode specifies the operation the instruction performs.
- It tells the processor what action to take.
- For LDA, the opcode indicates loading a value into the accumulator.

SIC vs. SIC/XE

SIC (Symbolic Instruction Code) is a basic assembly language with limited features.
- SIC/XE extends SIC with advanced features, such as:
  - Extended Instruction Set (XE): Additional instructions (e.g., floating-point operations).
  - Extended Addressing Modes: More flexible addressing options (e.g., base relative, indexed).

Operand Format in LDA Instruction

The operand '3600 = 0 011 0110 0000 0000' represents a 15-bit address in SIC/XE.
- The first 3 bits (0 011) represent the addressing mode, indicating direct addressing in this case.
- The remaining 12 bits (0110 0000 0000 0000) represent the actual address.

Assembler Type

The SIC/XE machine code requires a two-pass assembler.
- Two-pass assemblers process the source code twice to resolve address dependencies and generate the object code.

EQU Directive

The EQU directive assigns a constant value to a symbolic name.
- Useful for defining symbolic constants and making programs more readable.

Forward References in One-Pass Assembly

In a one-pass assembler, forward references (using a label before it is defined) are handled using a forward reference table.
- The table stores the information about forward references, and the assembler uses the table to resolve the addresses in the second pass.

Assignment Directive

The ORG directive is used to assign values to names in a program.
- It sets the location counter (LOCCTR) to a specific address.

LTORG Statement

The LTORG statement indicates that the assembler should generate literals at the current location.
- Literals are constants used in the program.

LOCCTR

LOCCTR (Location Counter) keeps track of the current address during assembly.
- Used to determine the memory address for each instruction and data.

Load-and-Go Assembler Object Program Generation

A load-and-go assembler combines the assembly and loading steps into one.
- It generates the object code directly and loads it into memory for execution.

START Directive in Pass2

The START directive is used in Pass 2 to determine the starting address of the program.
- It sets the initial value of LOCCTR.

PC-relative vs. B-relative Addressing

PC-relative Addressing: The operand address is relative to the program counter (PC), which points to the current instruction.
B-relative Addressing: The operand address is relative to the base register (B), which holds a base address determined by the programmer.

Program Loaded at Specific Address

When a program is loaded at a specific address, all addresses within the program are relocated to match the new starting address.
- This relocation ensures the program runs correctly in its new memory location.

OPTAB

OPTAB (Operation Table) is a table containing the opcodes and their corresponding machine language representations.
- The assembler uses OPTAB to translate symbolic instructions into machine code.

One-Pass vs. Multi-Pass Assembler

One-pass assembler: Processes the source code only once.
Multi-pass assembler: Processes the source code multiple times to completely resolve address dependencies and generate the object code.
- A two-pass assembler is a common type of multi-pass assembler.

Symbol Table (SYMTAB)

SYMTAB (Symbol Table) stores the symbolic names and their corresponding addresses in the program.
The assembler uses SYMTAB to resolve symbol references during assembly.

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Description

This quiz covers the encoding and interpretation of SIC/XE machine instructions. It includes examples of different instruction formats, opcode meanings, and calculation of effective addresses.