The DEFAULT directive changes the assembler defaults. Normally, Yasm defaults to a mode where the programmer is expected to explicitly specify most features directly. However, sometimes this is not desirable if a certain behavior is very commmonly used.

Currently, the only DEFAULT that is settable is whether or not registerless effective addresses in 64-bit mode are RIP-relative or not. By default, they are absolute unless overridden with the REL specifier (see Section 3.3). However, if DEFAULT REL is specified, REL is default, unless overridden with the ABS specifier, a FS or GS segment override is used, or another register is part of the effective address.

The special handling of FS and GS overrides are due to the fact that these segments are the only segments which can have non-0 base addresses in 64-bit mode, and thus are generally used as thread pointers or other special functions. With a non-zero base address, generating RIP-relative addresses for these forms would be extremely confusing. Other segment registers such as DS always have a base address of 0, so RIP-relative access still makes sense.

DEFAULT REL is disabled with DEFAULT ABS. The default mode of the assembler at start-up is DEFAULT ABS.