TYPES OF JUMP

The three types of jump, near, short, and far, differ in the size of instruction and the range of memory they can jump to with the smallest short form of two bytes and a range of just 256 bytes to the far form of five bytes and a range coveri ng the whole memory.

Short Jump

EB Disp

Near Jump

EB Disp Low Disp High

Far Jump

EB IP Low IP High CS Low CS High



Near Jump

When the relative address stored with the instruction is in 16 bits as in the last example the jump is called a near jump. Using a near jump we can jump anywhere within a segment. If we add a large number it will wrap around to

42


the lower part. A negative number actually is a large number and works this way using the wraparound behavior.

Short Jump

If the offset is stored in a single byte as in 75F2 with the opcode 75 and operand F2, the jump is called a short jump. F2 is added to IP as a signed byte. If the byte is negative the complement is negated from IP otherwise the byte is added. Unconditional jumps can be short, near, and far. The far type is yet to be discussed. Conditional jumps can only be short. A short jump can go +127 bytes ahead in code and -128 bytes backwards and no more. This is the limitation of a byte in singed representation.

Far Jump

Far jump is not position relative but is absolute. Both segment and offset must be given to a far jump. The previous two jumps were used to jump within a segment. Sometimes we may need to go from one code segment to another, and near and short jumps cannot take us there. Far jump must be used and a two byte segment and a two byte offset are given to it. It loads CS wit the segment part and IP with the offset part. Execution therefore resumes from that location in physical memory. The three instructions that have a far form are JMP, CALL, and RET, are related to program control. Far capability makes intra segment control possible.