Group-Base Address Mask Registers; Chip-Select Registers - Motorola DragonBall MC68328 User Manual

in the group-base mask register define the block size for the group. The base address field
is compared to the address on the address bus to determine if the group is decoded.
V Valid Bit
This bit indicates the validity of the contents of its base address register and address mask
register pair. The programmed chip-selects do not assert until the V-bit is set. A reset
clears the V-bit in each base address register.
1 = Contents are valid
0 = Contents are not valid

2.5.2 Group-Base Address Mask Registers

These registers adjust the comparison range for the group. Bits in this register set to 1
always compare true with the corresponding address line.
15 14 13 12
GMA31 GMA30 GMA29 GMA28 GMA27 GMA26 GMA25 GMA24 GMA23 GMA22 GMA21 GMA20
Address: $(FF)FFF108, 10A, 10C, 10E
Figure 2-10. Group-Base Address Mask Registers
GROUP BASE MASK (GMA31-GMA20)
0 = The corresponding address bit must match for a group to match
1 = The corresponding address bit compares true (don't care)
These bits mask address A31 through A20. Usually, the group base address mask register
bits select the group size. For example, if all the mask bits are cleared, A31 to A20 are com-
pared against the value programmed in the group base address register. In this case, the
group has a 1 Mbyte space. If bit 4 (GMA20 bit) is set and the remainder of the bits are clear,
the group is selected if A31 to A21 are the same as the value programmed in the group-base
address register. This provides 2 Mbyte of space for the group. Further decoding is per-
formed for each chip-select by comparing lower address lines and the chip-select registers.
If the devices are small, it is possible to program the groups to the same space and use the
chip-select registers to decode the areas for each chip-select.

2.5.3 CHIP-SELECT REGISTERS

There are four 32-bit chip-select option registers in each chip-select group—one for each
chip-select signal.
31 30 29 28
ADDRESS COMPARE A23-A16
AC23 AC22 AC21 AC20
15 14 13 12
ADDRESS MASK 23-16
AM23 AM22 AM21 AM20
Address: $(FF)FFF110, 114, 118, 11C, 120, 124, 128, 12C
Figure 2-11. Chip-Select Registers for Group A and B
MOTOROLA MC68328 DRAGONBALL PROCESSOR USER'S MANUAL
11 10 9 8
GROUP BASE MASK (GMA31-GMA20)
27 26 25 24
AC19 AC18 AC17 AC16
11 10 9 8
AM19 AM18 AM17 AM16
System Integration Module
7 6 5 4
23 22 21 20
UNUSED
- - - -
7 6 5 4
UNUSED
- - - -
3 2 1 0
UNUSED
0 0 0 0
Reset Value: $0000
19 18 17 16
BSW
- - - -
3 2 1 0
RO WAIT
- - - -
Reset Value: $00010006
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