80C186EB/80C188EB Microprocessor User's Manual - CEUNES

PERIPHERAL CONTROL BLOCK4.4.3.1 Writing the PCB Relocation RegisterWhenever mapping the Peripheral Control Block to another location, the user should program theRelocation Register with a byte write (i.e., OUT DX, AL). Internally, the Relocation Register iswritten with 16 bits of the AX register, while externally the Bus Interface Unit runs a single 8-bitbus cycle. If a word instruction (i.e., OUT DX, AX) is used with an 80C188 Modular Core familymember, the Relocation Register is written on the first bus cycle. The Bus Interface Unit then runsan unnecessary second bus cycle. The address of the second bus cycle is no longer within the controlblock, since the Peripheral Control Block was moved on the first cycle. External READYmust now be generated to complete the cycle. For this reason, we recommend byte operations forthe Relocation Register.4.4.3.2 Accessing the Peripheral Control RegistersByte instructions should be used for the registers in the Peripheral Control Block of an 80C188Modular Core family member. This requires half the bus cycles of word operations. Byte operationsare 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.4.4.3.3 Accessing Reserved LocationsUnused locations are reserved. If a write is made to these locations, a bus cycle occurs, but datais 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 becleared to 0H.NOTEFailure to follow this guideline could result in incompatibilities with future80C186 Modular Core family products.4.5 SETTING THE PCB BASE LOCATIONUpon 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 to0FFFFH). Writing the PCB Relocation Register allows the user to change that location.4-6