The Pascal convention was used by OS/2 1.x for those calls which access
system functions which are implemented in a higher privilege level (ring)
than the application. It is also used to call 16 bit Window Procedures.
Two examples are DosAllocSeg and DosRead. The decision was made to use
the Pascal convention because of the way the hardware protects access to
instructions and storage which is more privileged. This type of interface,
including hardware operation, is discussed in detail after basic stack operation
has been discussed.