🚀 go-pugleaf

In article <394@houxs.UUCP>, daw@houxs.UUCP (D.WOLVERTON) writes:

> In some systems, the hardware floating point (fp) unit is _optional_.
> The Itty Bitty Machines (IBM) PC is a good example.  From the

> 3)  Code generation pretends that the fp unit will always be present,
> so it emits code which uses the fp unit directly into the instruction
> stream.  If a fp unit is not present, the hardware arranges for a trap
> to occur which transfers control to the OS.  At this point either:

> 4)  Code generation emits code which always causes transfer to the
> OS, e.g. by illegal opcodes or TRAP instructions.  The OS then

With the *86/*87 chips, there is a very elegant solution combining
3 & 4.  Both emulation and real code are easily defined to be
identical except by a constant difference in the first byte.
Then the OS can select for the presence or absence of the chip at
program load time by using a relocation table to modify the initial
instruction bytes if the *87 is absent.  This way you get optimal
emulator performance and optimal hardware performance.
-- 
Stuart D. Gathman	<..!seismo!dgis!bms-at!stuart>