!lm12
!rm75
Potential Trouble in TYMAC...................Robert H. Bernard

[ Bob is a subscriber in Westport, Connecticut. ]

The article by Peter Bartlett on improving the Epson Controller Card (which appeared in the February 1982 issue of AAL) has prompted me to write to bring to the attention of fellow AAL readers that the TYMAC controller card, which is a lower-cost alternative to the official Epson card, has a potentially serious problem.

To achieve slot independence, controller card ROM programs JSR to an RTS instruction in the Monitor.  Then they extract the slot from the return address the JSR put on the stack.  The Apple II Reference Manual details the process on page 81-82.

Most controller cards use the Apple technique verbatim, JSR'ing to $FF58, which is an RTS instruction in the Monitor ROM.  However, the TYMAC card JSR's to $FDFF.   That location also contains an RTS, so there is no problem using the TYMAC card as long as the Monitor ROM is enabled.

The problem occurs when the TYMAC card is used with Pascal.  While Apple Computer has specifically guaranteed an RTS instruction at $FF58 in the Pascal Basic Input/Output System (BIOS), no RTS exists at $FDFF.  Therefore TYMAC loses control and causes a Pascal crash as soon as it is called.

If any of you have TYMAC cards, and plan to make the Peter Bartlett modification (or perhaps even if you don't plan to), you should also change the JSR instruction at $0A relative to the beginning of the ROM from 20FFFD to 2058FF.
