This article describes the IBM chipset shared memory register on the Apple Token Ring NB/c Card.

The definitions and addresses of the Shared Memory Control Registers on the IBM module are shown below. Each register is 16 bits, with the upper byte being addressed by an even address (for example 81E00) and the lower byte being addressed by an odd address (for example 81E01). Either byte or word accesses may be used. Refer to section 6.6 of the "Token-Ring Mini-Card Technical Reference" for explanations of these registers. Chapter 7 of the "IBM Local Area Network Technical Reference" has additional information, though the Mini-Card specification supersedes it in the event of a discrepancy. Also note that the bit ordering is different between these two manuals; The former labels the most significant bit as bit "15", while the latter calls the same bit "0".

Address  Function
-------  --------
81E00    RRR  - Shared RAM Relocation Register (set upper byte to "10")
81E02    WRBR - Write Region Base Address Register
81E04    WWOR - Write Window Open Register
81E06    WWCR - Write Window Close Register
81E08    UISR (ISRP) - User Interrupt Status Register  (adapter-to-host)

81E0A    LISR (ISRA) - Adapter Interrupt Status Register (host-to-adapter)
81E0C    TCR  - Timer Control Register
81E0E    TVR  - Timer Value Register
81E10    UER  - Reserved
81E12    SRR  - Soft Reset Register
81E14    IVR  - Interrupt Vector Register (not used)
81E16    JR   - Jumper Register
81E18    SRPR - Shared RAM Page Register (set upper byte to "C0")

The above registers are duplicated 3 times, for a total of 4 banks of registers. The registers present in each bank are identical, however the operation performed changes based on the bank addressed. The following shows the address ranges of each bank, and the resulting operation.

Range         Operation
-----         ---------
81E00-81E1F   Read or Write
81E20-81E3F   Read/Reset under mask (0)
81E40-81E5F   Read/Set under mask (1)
81E60-81E7F   Read only