Execution context isolation during the DXE phase of computer start-up. Provision is made for referencing and dereferencing execution contexts and thereby providing execution isolation across contexts. In response to invoking a BIOS kernel function during a Driver Execution Environment (DXE) phase of a boot-up of the computer, the BIOS kernel associates a first processor context with the sequence of instructions, determines that scheduling the sequence of instructions requires a switch from a second processor context to the first processor context, performs a context switch action, during the DXE phase of the boot-up of the computer, to switch from the second processor context to the first processor context.