AMD Am186 ES, User Manual, Page: 76 / 192

Chip Select Unit 5-6
5.5.2 Low Memory Chip Select Register
(LMCS, Offset A2h)
The Am186ES and Am188ES microcontrollers provide the LCS chip select pin for the
bottom of memory. Since the interrupt vector table is located at 00000h at the bottom of
memory, the LCS pin has been provided to facilitate this usage. The LCS pin is not active
on reset, but any write access to the LMCS register activates this pin.
Before activating the LCS chip select, the width of the data bus for LCS space should be
configured in the AUXCON register.
The Low Memory Chip Select is configured through the LMCS register (see Figure 5-2).
Figure 5-2 Low Memory Chip Select Register
The value of the LMCS register at reset is undefined except DA is set to 0.
Bit 15: Reserved—Set to 0.
Bits 14–12: Upper Boundary (UB2–UB0)—The UB2–UB0 bits define the upper boundary
of the memory accessed through the LCS chip select. Because of the timing requirements
of the LCS output and the nonmultiplexed address bus, the number of programmable
memory sizes for the LMCS register is reduced compared to the 80C186 and 80C188
microcontrollers. Consequently, the number of programmable bits has been reduced from
eight bits in the 80C186 and 80C188 microcontrollers to three bits in the Am186ES and
Am188ES microcontrollers.
The Am186ES and Am188ES microcontrollers have a block size of 512 Kbytes, which is
not available on the 80C186 and 80C188 microcontrollers. Table 5-3 outlines the possible
configurations.
Table 5-3 LMCS Block Size Programming Values
Memory Block
Size
Ending
Address UB2–UB0
64K 0FFFFh 000b
128K 1FFFFh 001b
256K 3FFFFh 011b
512K 7FFFFh 111b
1
15
7 0
0
UB2–UB0
1 1 1
PSE
1 1 1
A19
R1–R0 R2
DA