Gixen main vs mirror offset
With a maximum of 127 bytes in-between them. They use relative offset values from 00h to 7Fh which enable program execution to jump to another instruction If you use a SHORT directive in your sourceĪnd the address ends up being too far away for a SHORT Jump, you'll get an error message.Įasiest of the two to work with. Instruction) that label name actually points to.
Gixen main vs mirror offset code#
With only a label name, Assemblers need more than one pass through your source code to know how far away (from a Jump
Though it might not be necessary unless you include a " SHORT" directive before the "JMP" mnemonic in your source code! This may explain why you see a "NoOp" (90h) byte after a SHORT Jump in code thatĭoesn't need an extra byte. Most Assemblers, however, will create space for at least a 3-byte NEAR Jump even The main reason it can do this is because the exact location of the next instruction must be specified. * Note: MS-DEBUG's Assembler will use the smallest possible JMP code (first SHORT, then NEAR and finallyįAR) for any address you give it. Therefore, programmers who are trying to keep a routine down to the least number of bytes, must know the limits ofīoth Forward and Reverse SHORT (and NEAR) Jumps! Jump instead * (an Absolute FAR Jump is one that will jump outside of the present 64 KiBĬode Segment). If you point to anĪddress that's too far away for a SHORT Jump to reach, the Assembler should code the instruction as a three-byte NEAR Required by MS- Debug's Assemble command), all Assemblers still figure out the value of the offset byte for you. Whether you use a label to point to the next instruction or a specific address (as The first byte ofĪ SHORT Jump is always EB and the second is a relative offset from 00h to 7Fh for Forward jumps, andįrom 80h to FFh for Reverse (or Backward) jumps. Using only Relative Jump instructions can be relocated anywhere in memory without having to change the machine code for the Jumps. These are also known as SHORT Relative Jumps. Jumps (such as JE, JG, JC, JZ, JNE, JNG, JNC, JNZ, etc.) as well! Though we mention only JMP code, what you'll learn here about Relative offsets will also apply to all Conditional Here we discuss the use of two-byte JMP instructions in x86