far jmp?

In real-mode 16bit asm, addresses can be local to a segment or include segment information. A jump involving an address with segment+offset information is a far jump.

It is irrelevant in 32bit flat mode.

Note: that''s what those ''FAR'' macros stand for in the windows API.

Documents [ GDNet | MSDN | STL | OpenGL | Formats | RTFM | Asking Smart Questions ]
C++ Stuff [ MinGW | Loki | SDL | Boost. | STLport | FLTK | ACCU Recommended Books ]

quote:

---

It is irrelevant in 32bit flat mode.

---

Well, that''s generally true, but for other architecture''s far jump takes on another meaning (MIPS esp.)

You can jump two different ways:
j Address
or
jr Register

using the j address, the instruction can only be 32 bits, so the top 6 bits are the op-code, and the bottom 26 bits are the address to jump to (shifted left twice, since it must be on 4-byte boundary). This means, if a jump is occuring from an address over 28 bits away from the current PC, you can''t use j Address.

You can use jr Register to jump to the address stored in the register since it is 32-bits regadless.

This is what the #pragma far_call on/off is used for in C compilation code.

Just a lesson on other architectures.

thanks. I am on the x86, but its not in flat mode. I need to do the far jump from a 16 bit segment to a 32 bit(writing bootloader)

A pmode far jump (needed to get into 32 bit mode) loads a segment selector into cs and jumps to the specified offset in this segment.
Code: jmp dword sel:ofs (opcode 0xea)