Intel 80C186EA, User Manual

Page: 137 / 417

PERIPHERAL CONTROL BLOCK
4-6
4.4.3.1 Writing 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.
4.4.3.2 Accessing 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.
4.4.3.3 Accessing 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.
4.5 SETTING 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.