Apple IIgs #32
/INH Line Anomaly

Revised by Glenn A. Baxter & Rob Moore (November 1988)
Written by Glenn A. Baxter (December 1986)

This Technical Note describes a hardware anomaly which affects the use of the /INH line on the Apple IIGS.


The Apple IIGS maps logical addresses in main and auxiliary RAM spaces to physical RAM devices in such a way that using the /INH line can cause bus contention under certain conditions. This Note describes the problem and suggests a solution strategy.

In the Apple IIGS, main memory resides within four physical 64 x 4 DRAMs. Memory is logically mapped into two separate banks of 64K x 8. The logical map of main memory is slightly different than what one might expect. Owing to the demands of new video modes on the IIGS, the DRAMs need a greater amount of time to perform their function. The easiest way to allocate time in a fixed, time-based system is to use a memory interleaving mechanism, and the IIGS implements its video in this fashion.

As a result of this interleaving scheme, the logical map of main and auxiliary memory does not correspond directly to physical DRAMs, but are split in three places. The split looks like the following:


    First Physical 64K                   Second Physical 64K
    Main Memory         $0000 - $5FFF    Auxiliary Memory    $0000 - $5FFF
    Auxiliary Memory    $6000 - $9FFF    Main Memory         $6000 - $9FFF
    Main Memory         $A000 - $FFFF    Auxiliary Memory    $A000 - $FFFF

Only part of the first physical bank of RAM is inhibited when /INH is brought low; therefore, the /INH function only works between $0000 - $5FFF and $A000 - $FFFF in main memory and $6000 - $6FFF in auxiliary memory. If a card attempts to inhibit main memory in the range of $6000 - $9FFF or auxiliary memory in the ranges $0000 - $5FFF or $A000 - $FFFF, bus contention results as both the Mega II and the 74HCT245 buffer device attempt to drive the bus simultaneously (which can damage the Mega II).

Because earlier Apple II systems do not arrange their physical memory as described above, cards which use the /INH line may be compatible with the Apple ][+ and IIe, but not with the IIGS. To be compatible with all Apple II systems, a card should include an address mask that will prevent /INH from going low when the address in in the sensitive ranges of main or auxiliary memory. The following logic equation represents an appropriate blocking signal for main memory inhibition:


    BLOCK    =    /A15    *    A14    *    A13    ;BLOCK $6000-$7FFF
             +    A15    *    /A14    *    /A13   ;BLOCK $8000-$9FFF


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