How To Force/specify A Long Jump In Fasm?

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Introduction

In assembly language programming, particularly in 16-bit real mode, jumps can be either short or long. A short jump is a jump within the range of -128 to 127 bytes from the current instruction, while a long jump is a jump beyond this range. In this article, we will explore how to force or specify a long jump in FASM (Flat Assembler), a popular assembly language assembler.

Understanding Jumps in 16-bit Real Mode

In 16-bit real mode, the CPU uses a segmented memory model. The segment registers (CS, DS, ES, FS, GS, and SS) are used to access different parts of memory. The CS (Code Segment) register is used to access the code segment, which contains the program instructions. The IP (Instruction Pointer) register points to the current instruction being executed.

When a jump instruction is executed, the CPU updates the IP register to point to the target address of the jump. However, if the target address is beyond the range of a short jump, a long jump is required.

Short Jumps vs. Long Jumps

A short jump is a jump within the range of -128 to 127 bytes from the current instruction. The short jump instruction (JB, JC, JE, JG, JL, JGE, JLE, JNGE) uses an 8-bit signed displacement value to calculate the target address.

On the other hand, a long jump is a jump beyond the range of a short jump. The long jump instruction (JMP) uses a 16-bit signed displacement value to calculate the target address.

Forcing a Long Jump in FASM

To force a long jump in FASM, you can use the JMP instruction with a 16-bit signed displacement value. The syntax for the JMP instruction is as follows:

JMP label

However, if you want to specify a long jump with a specific displacement value, you can use the following syntax:

JMP offset:16

In this syntax, offset is the 16-bit signed displacement value, and :16 specifies that the value is a 16-bit value.

Example: Long Jump with a Specific Displacement Value

Let's consider an example where we want to jump to a label located at an address that is beyond the range of a short jump. We can use the following code:

org 0xF0000

start:
mov ah, 0x0E
mov al, 'L'
int 0x10


; Jump to a label located at address 0x1000
JMP 0x1000:16



label:
; This label is located at address 0x1000
mov ah, 0x0E
mov al, 'A'
int 0x10

In this example, we use the JMP instruction with a 16-bit signed displacement value to jump to the label located at address 0x1000.

Conclusion

In conclusion, forcing or specifying a long jump in FASM is a straightforward process. You can use the JMP instruction with a 16-bit signed displacement to force a long jump. Additionally, you can use the JMP offset:16 syntax to specify a long jump with a specific displacement value. By following the examples and syntax provided in this article, you should be able to force or specify a long jump in FASM with ease.

Tips and Variations

  • To jump to a label located at a specific address, use the JMP label syntax.
  • To specify a long jump with a specific displacement value, use the JMP offset:16 syntax.
  • Make sure to use the correct syntax and syntax for the JMP instruction to avoid errors.
  • You can use the JMP instruction to jump to a label located at any address in the code segment.

Common Errors and Solutions

  • Error: "Invalid instruction" or "Invalid operand"
  • Solution: Check the syntax and syntax for the JMP instruction. Make sure to use the correct syntax and syntax for the JMP instruction.
  • Error: "Jump out of range"
  • Solution: Check the displacement value used in the JMP instruction. Make sure the displacement value is within the range of a long jump.

FASM Syntax Reference

  • JMP label: Jump to a label located at a specific address.
  • JMP offset:16: Specify a long jump with a specific displacement value.

FASM Assembler Reference

  • org 0xF0000: Set the origin of the code segment to address 0xF0000.
  • mov ah, 0x0E: Move the value 0x0E into the AH register.
  • mov al, 'L': Move the character 'L' into the AL register.
  • int 0x10: Interrupt 0x10.

FASM Code Example

org 0xF0000

start:
mov ah, 0x0E
mov al, 'L'
int 0x10


; Jump to a label located at address 0x1000
JMP 0x1000:16



label:
; This label is located at address 0x1000
mov ah, 0x0E
mov al, 'A'
int 0x10

FASM Online Resources

Q: What is a long jump in FASM?

A: A long jump in FASM is a jump instruction that uses a 16-bit signed displacement value to calculate the target address. It is used to jump to a label located at an address that is beyond the range of a short jump.

Q: How do I specify a long jump in FASM?

A: To specify a long jump in FASM, you can use the JMP instruction with a 16-bit signed displacement value. The syntax for the JMP instruction is as follows:

JMP offset:16

In this syntax, offset is the 16-bit signed displacement value, and :16 specifies that the value is a 16-bit value.

Q: What is the difference between a short jump and a long jump in FASM?

A: A short jump in FASM is a jump within the range of -128 to 127 bytes from the current instruction. The short jump instruction (JB, JC, JE, JG, JL, JGE, JLE, JNGE) uses an 8-bit signed displacement value to calculate the target address.

On the other hand, a long jump is a jump beyond the range of a short jump. The long jump instruction (JMP) uses a 16-bit signed displacement value to calculate the target address.

Q: Can I use the JMP instruction to jump to a label located at a specific address?

A: Yes, you can use the JMP instruction to jump to a label located at a specific address. The syntax for the JMP instruction is as follows:

JMP label

In this syntax, label is the label located at the specific address.

Q: How do I specify a long jump with a specific displacement value in FASM?

A: To specify a long jump with a specific displacement value in FASM, you can use the following syntax:

JMP offset:16

In this syntax, offset is the 16-bit signed displacement value, and :16 specifies that the value is a 16-bit value.

Q: What is the origin of the code segment in FASM?

A: The origin of the code segment in FASM is set using the org directive. The syntax for the org directive is as follows:

org 0xF0000

In this syntax, 0xF0000 is the origin of the code segment.

Q: How do I set the origin of the code segment in FASM?

A: To set the origin of the code segment in FASM, you can use the org directive. The syntax for the org directive is as follows:

org 0xF0000

In this syntax, 0xF0000 is the origin of the code segment.

Q: What is the difference between the JMP instruction and the JB, JC, JE, JG, JL, JGE, JLE, and JNGE instructions in FASM?

A: The JMP instruction is a long jump instruction that uses a 16-bit signed displacement value to calculate the target address. On the other hand, the JB, JC, JE, JG, JL, JGE, JLE, and JNGE instructions are short jump instructions that use an 8-bit signed displacement value to calculate the target address.

Q: Can I use the JMP instruction to jump to a label located at an address that is beyond the range of a short jump?

A: Yes, you can use the JMP instruction to jump to a label located at an address that is beyond the range of a short jump. The JMP instruction uses a 16-bit signed displacement value to calculate the target address, which allows it to jump to addresses beyond the range of a short jump.

Q: How do I specify a long jump with a specific displacement value in FASM?

A: To specify a long jump with a specific displacement value in FASM, you can use the following syntax:

JMP offset:16

In this syntax, offset is the 16-bit signed displacement value, and :16 specifies that the value is a 16-bit value.

Q: What is the difference between the JMP instruction and the CALL instruction in FASM?

A: The JMP instruction is a long jump instruction that uses a 16-bit signed displacement value to calculate the target address. On the other hand, the CALL instruction is a procedure call instruction that uses a 16-bit signed displacement value to calculate the target address.

Q: Can I use the JMP instruction to call a procedure in FASM?

A: No, you cannot use the JMP instruction to call a procedure in FASM. The JMP instruction is a long jump instruction that is used to jump to a label located at a specific address, while the CALL instruction is a procedure call instruction that is used to call a procedure.

Q: How do I call a procedure in FASM?

A: To call a procedure in FASM, you can use the CALL instruction. The syntax for the CALL instruction is as follows:

CALL label

In this syntax, label is the label of the procedure to be called.

Q: What is the difference between the JMP instruction and the RET instruction in FASM?

A: The JMP instruction is a long jump instruction that uses a 16-bit signed displacement value to calculate the target address. On the other hand, the RET instruction is a return instruction that returns control to the previous procedure.

Q: Can I use the JMP instruction to return from a procedure in FASM?

A: No, you cannot use the JMP instruction to return from a procedure in FASM. The JMP instruction is a long jump instruction that is used to jump to a label located at a specific address, while the RET instruction is a return instruction that returns control to the procedure.

Q: How do I return from a procedure in FASM?

A: To return from a procedure in FASM, you can use the RET instruction. The syntax for the RET instruction is as follows:

RET

In this syntax, there is no operand for the RET instruction.

Q: What is the difference between the JMP instruction and the INT instruction in FASM?

A: The JMP instruction is a long jump instruction that uses a 16-bit signed displacement value to calculate the target address. On the other hand, the INT instruction is an interrupt instruction that generates an interrupt at the specified address.

Q: Can I use the JMP instruction to generate an interrupt in FASM?

A: No, you cannot use the JMP instruction to generate an interrupt in FASM. The JMP instruction is a long jump instruction that is used to jump to a label located at a specific address, while the INT instruction is an interrupt instruction that generates an interrupt at the specified address.

Q: How do I generate an interrupt in FASM?

A: To generate an interrupt in FASM, you can use the INT instruction. The syntax for the INT instruction is as follows:

INT 0x10

In this syntax, 0x10 is the interrupt number.

Q: What is the difference between the JMP instruction and the IRET instruction in FASM?

A: The JMP instruction is a long jump instruction that uses a 16-bit signed displacement value to calculate the target address. On the other hand, the IRET instruction is an interrupt return instruction that returns control to the previous procedure after an interrupt.

Q: Can I use the JMP instruction to return from an interrupt in FASM?

A: No, you cannot use the JMP instruction to return from an interrupt in FASM. The JMP instruction is a long jump instruction that is used to jump to a label located at a specific address, while the IRET instruction is an interrupt return instruction that returns control to the previous procedure after an interrupt.

Q: How do I return from an interrupt in FASM?

A: To return from an interrupt in FASM, you can use the IRET instruction. The syntax for the IRET instruction is as follows:

IRET

In this syntax, there is no operand for the IRET instruction.

Q: What is the difference between the JMP instruction and the LGDT instruction in FASM?

A: The JMP instruction is a long jump instruction that uses a 16-bit signed displacement value to calculate the target address. On the other hand, the LGDT instruction is a load global descriptor table instruction that loads the global descriptor table from the specified address.

**Q: Can I use the JMP