Introduction

The MacVAX simulator provides a collection of built-in "system" calls using the extended function call (xfc) instruction. The original intent was to provide an easy method for performing tasks normally performed by an operating system, or provided as library calls, without the need to provide an OS running on the simulator. This was later extented to allow an OS running on MacVAX, to interact with the simulator environment.

The current version of the simulator does run a VAX OS which uses the chmk instruction to provide system calls which replace many of these xfc calls. The earlier xfc calls have been retained for use by introductory courses which do not want to have the complications introduced by an OS.

The general format of an xfc call is:

xfc n where n is a byte representing the xfc call number. Arguments are passed to and returned from xfc calls in registers.

Example:

To read one character from the keyboard, we use xfc call number 0, and the character read gets returned in r0. xfc #0; Read one character (returned in r0) Alternatively we could use another register for the xfc call number. movl #0, r2 xfc r2 ; Read one character (returned in r0) The xfc calls can be refered to by name if xfc.h is included: .include "xfc.h" xfc #READCHAR ; Read one character (returned in r0)

Nb. The assembler recognises sys as a subtituted for xfc, but the simulator will always display these as an xfc.

Passing an xfc call an invalid argument will result in an reserved operand fault.