Intel 80C186XL, 80C188XL Setting the PCB Base Location, Writing the PCB Relocation Register

PERIPHERAL CONTROL BLOCK

4.4.3.1Writing the PCB Relocation Register

Whenever mapping the Peripheral Control Block to another location, the user should program the Relocation Register with a byte write (i.e., OUT DX, AL). Internally, the Relocation Register is written with 16 bits of the AX register, while externally the Bus Interface Unit runs a single 8-bit bus cycle. If a word instruction (i.e., OUT DX, AX) is used with an 80C188 Modular Core family member, the Relocation Register is written on the first bus cycle. The Bus Interface Unit then runs an unnecessary second bus cycle. The address of the second bus cycle is no longer within the con- trol block, since the Peripheral Control Block was moved on the first cycle. External READY must now be generated to complete the cycle. For this reason, we recommend byte operations for the Relocation Register.

4.4.3.2Accessing the Peripheral Control Registers

Byte instructions should be used for the registers in the Peripheral Control Block of an 80C188 Modular Core family member. This requires half the bus cycles of word operations. Byte opera- tions are valid only for even-addressed writes to the Peripheral Control Block. A word read (e.g., IN AX, DX) must be performed to read a 16-bit Peripheral Control Block register when possible.

4.4.3.3Accessing Reserved Locations

Unused locations are reserved. If a write is made to these locations, a bus cycle occurs, but data is not stored. If a subsequent read is made to the same location, the value written is not read back. If reserved registers are written (for example, during a block MOV instruction) they must be cleared to 0H.

NOTE

Failure to follow this guideline could result in incompatibilities with future 80C186 Modular Core family products.

4.5SETTING THE PCB BASE LOCATION

Upon reset, the PCB Relocation Register (see Figure 4-1 on page 4-2) contains the value 00FFH, which causes the Peripheral Control Block to be located at the top of I/O space (0FF00H to 0FFFFH). Writing the PCB Relocation Register allows the user to change that location.

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