The coprocessor accesses memory as the CPU does, but it has its own data and control registers––eight data registers organized as a stack and seven control registers similar to the 8086 flag registers. The coprocessor's instruction set provides direct access to these registers.
Summary: The eight coprocessor data registers form a stack.
The eight 80-bit data registers of the 8087-based coprocessors are organized as a stack although they need not be used as a stack. As data items are pushed into the top register, previous data items move into higher-numbered registers, which are lower on the stack. Register 0 is the top of the stack; register 7 is the bottom. The syntax for specifying registers is shown below:
ST [[(number)]]
The number must be a digit between 0 and 7 or a constant expression that evaluates to a number from 0 to 7. ST is another way to refer to ST(0).
All coprocessor data is stored in registers in the 10-byte real format. The registers and the register format are shown in Figure 6.2.
Internally, all calculations are done on numbers of the same type. Since 10-byte real numbers have the greatest precision, lower-precision numbers are guaranteed not to lose precision as a result of calculations. The instructions that transfer values between the main memory and the coprocessor automatically convert numbers to and from the 10-byte real format.