Assembly Language Jump Instructions Quiz

Questions and Answers

What is the purpose of jump instructions in assembly language programming?

The purpose of jump instructions in assembly language programming is to perform branching or looping based on conditional or unconditional statements.

How does a jump instruction in assembly language differ from a 'go to' statement in C?

A jump instruction in assembly language is similar to a 'go to' statement in C, but it does not push a return address via %esp.

What design technique is best suited for designing an assembly language program?

The flowchart design technique is best suited for designing an assembly language program.

What is the purpose of the 'subb' instruction in the Gas code?

The 'subb' instruction is used to perform a subtraction and set/reset the sign flag. Signup and view all the answers

What is the difference between the 'jl' and 'jle' conditional jumps?

The 'jl' jump is taken if the less than condition is true, while the 'jle' jump is taken if the less than or equal condition is true. Signup and view all the answers

What is the significance of the carry flag in unsigned comparisons?

The carry flag indicates underflow in unsigned comparisons. If underflow occurs, the carry flag is set to 1, indicating a negative result. Signup and view all the answers

What is the purpose of the 'loop' instruction?

The 'loop' instruction is used to decrement, test, and jump within a loop. It decrements the %ecx register and jumps to a specified label if %ecx is not equal to 0. Signup and view all the answers

What is the purpose of the 'jmp' instruction in assembly code?

The 'jmp' instruction is used to perform an unconditional jump to a specified location in the code. Signup and view all the answers

What is the difference between a conditional and unconditional jump in assembly code?

A conditional jump is dependent on the state of specific flags, and the jump is performed if the conditions are met. An unconditional jump does not rely on any conditions and always jumps to a specified location in the code. Signup and view all the answers

What are the carry flag and overflow flag used for in assembly code?

The carry flag is set if the addition or subtraction of two numbers causes a carry out or borrow in at the most significant bit position. The overflow flag is set if the sum of two numbers with the sign bits off yields a result number with the sign bit on, or if the sum of two numbers with the sign bits on yields a result number with the sign bit off. Signup and view all the answers

What is the purpose of the 'jl' instruction in assembly code?

The 'jl' instruction is used to perform a jump if the 'sign flag' XOR 'overflow flag' is set. It is commonly used for signed comparisons. Signup and view all the answers

Study Notes

Assembly Language Programming III: Condition Codes and Jump Instructions

Jumps and Flow of Control

In assembly language, there are no "if-else", "for", "do", or "do … while" statements like in C.
Jump instructions are similar to "go to" statements and "call" instructions, but they don't push a return address via %esp.
The processor fetches and executes instructions in a cycle: fetch, execute, and increment %eip.
Jump instructions can change the flow of control by loading a different value into %eip.

Design Techniques

Flowcharts are a suitable design technique for assembly language programs, using circles for labels and arrows for go-to's.

Conditional Jumps

Conditional jumps test specific flags and load a new value into %eip if the condition is true.
Examples of conditional jumps: jz, js, jnz, jns, jl, jle, jg, jge, je, jncc.

Unconditional Jumps

Unconditional jumps always load a new value into %eip, using a hard-coded address or a label.

Infinite Loop

An infinite loop can be created using a single unconditional jump instruction.

Flags

Flags are set by arithmetic or logical instructions, affecting the behavior of conditional jump instructions.
Types of flags: Carry Flag, Zero Flag, Sign Flag, Parity Flag, Overflow Flag, Auxiliary Carry Flag.

Conditional Jump Operation

Conditional jumps test specific flags and load a new value into %eip if the condition is true.
Example: jz label (jump if zero flag is set).

Signed Comparisons

Signed comparisons use the sign flag to determine the result of a comparison.
Example: jl label (jump if signed comparison is less than).
The cmp instruction sets flags according to a subtraction, without overwriting the registers being compared.

Unsigned Comparisons

Unsigned comparisons use the carry flag to determine the result of a comparison.
Example: jb label (jump if unsigned comparison is below).
Unsigned comparisons have the opposite meaning of the carry flag compared to signed comparisons.

Loop Instruction

The loop instruction decrements %ecx, tests for zero, and jumps to a label if not zero.
Example: loop label (decrement and test %ecx, jump to label if not zero).

Increment and Decrement Instructions

Increment and decrement instructions (incl and decl) are useful inside loops, equivalent to post-increment or decrement in C.