PC/AT Recipe

Work in progress.

Register usage: SP is PSP, BP is RSP, DX is IP

Unlike z80 boot code, we don't check PS at each next call (we
do check RS though). It is the responsibility of every native
PSP-modifying word to call chkPS, . Also, chkPS, is a bit
different than in z80: it is parametrizable. The idea is that
we always call chkPS, before popping, telling the expected size
of stack. This allows for some interesting optimization. For
example, in SWAP, no need to pop, chkPS, then push, we can
chkPS and then proceed to optimized swapping in PS.

811 MBR bootloader             812-829 Boot code