Definitions - Intel i960 User Manual

Language Implementation
The standard i960 processor calling convention places an absolute
minimum overhead on simple, commonly called functions with few
parameters. This is done by passing information between the calling
function and the called function in the i960 architecture's global registers
as much as possible.

Definitions

call-preserved register
call-scratch register
The following list summarizes usage of the global registers
and the floating-point registers
g15
...
g0 g7
...
g8 g11
The register must have the same value
upon exit from a function as it did upon
entry to the function.
The register may have a different value
upon exit from a function than it did
upon entry to the function.
- .
fp0 fp3
These eight registers pass parameters into the
called function from the calling function. If the
return value of the function is four words or less
in size, then the return value is passed back to the
calling function in registers
function returns a long double and generates
code for the KB or SB processor and
compatibility with ic960 R2.0 is requested, then
registers through
g0 g7
These four registers may be used for parameter
passing in addition to
g0
parameter or a piece of a parameter is passed in
one of these registers, that register is considered
a call-scratch register. That register is
considered a call-preserved register otherwise. If
the called function can not be sure that a register
has had a parameter passed in it, then the register
must be considered a call-preserved register.
through
g0
through . If the
g0 g3
are call-scratch registers.
through . If a
g7
7
7-15