Operand Descriptors

Operand Descriptors

1. In addition to providing a mechanism for communication between high-level code generation routines, the operand descriptor type is an essential component of the interface between the high and low level code-generation modules.

2. The type used for operand descriptors should be flexible enough to handle several possibilities:

   registers

   Since access to values in registers is generally faster than access to memory, we would like the code we generate to keep intermediate results in registers whenever possible.

   memory locations

   Since we will eventually run out of registers, our compiler may have to store some intermediate results in memory. Even if this were not the case, other factors would make it necessary/desirable to have operand descriptors for memory locations.

   • when we process an expression like x+y we would like to produce something like:

     move	x,D1
     add	y,D1

     rather than

     move	x,D1
     move	y,D2
     add	D2,D1

   • To make this possible, the routine that "generates code" for the sub-expression "y" has to be able to simply return a descriptor for y where it resides in memory without producing any code.

   constants

   When processing an expression like x+1 we would like to produce the code:

   move	x,D1
   add	#1,D1

   rather than

   move	x,D1
   move	#1,D2
   add	D2,D1

Operand Descriptors