PRIMS.TXT

BenOS v1.0 code primitives (c) Benjamin Hoyt 1997

Internally, BenOS Forth is a subroutine threaded Forth designed for
speed and multitasking. Hence it uses the 386 hardware stack for the
return stack, and for a data stack pointer it uses EBP. Also, EBX
always holds the top item on the stack for a bit of stack cache.
Register ESI is reserved for the user variable table pointer for
speedy multitasking. Here is a register usage map:

eax	= scratch
ebx	= top item on stack
ecx	= scratch
edx	= scratch
esi	= reserved for user pointer in task switching
edi	= scratch
ebp	= data stack pointer (see below)
esp	= return stack pointer
 cs	= read-only code segment, starts at physical 0, and is 4gb in size
 ds	= read/write data segment, starts at physical 0, and is 4gb in size
 es	= fs = gs = ss = same data segment as ds

Please note that the phrase "1234 sp@" does NOT give you a pointer to
the 1234 on the stack. This is because of how we are handling sp@ and
sp! with EBX as the top-of-stack register. The phrase
"1234 sp@ cell-" gives you a pointer to the 1234 on the stack. The
return stack is not affected like this.