gcc.info: Stack Registers

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Registers That Form a Stack

   There are special features to handle computers where some of the
"registers" form a stack, as in the 80387 coprocessor for the 80386.
Stack registers are normally written by pushing onto the stack, and are
numbered relative to the top of the stack.
   Currently, GNU CC can only handle one group of stack-like registers,
and they must be consecutively numbered.
`STACK_REGS'
     Define this if the machine has any stack-like registers.
`FIRST_STACK_REG'
     The number of the first stack-like register.  This one is the top
     of the stack.
`LAST_STACK_REG'
     The number of the last stack-like register.  This one is the
     bottom of the stack.