Chapter 4: Theory of operation
69
register in the CPLD controls which page is active. On initial power-up, the BIOS
Control register is set to point at the top of the boot block flash, where the PC BIOS
resides. At a later time, software can select one of the other seven banks to execute
from. The BIOS Control register is reset on all reset types (hard, soft, and POR).
Figure 4-3
shows the memory overlay at the top of memory space.
Each Flash device has its own write protect mechanism. The Boot Block of the PC
BIOS can be re-written only if the write-protect jumper is installed on the board.
The user Flash is write protected by bit 7 (D7) in the BIOS Control register. A one in
this position disables write protection of the device. The next table shows the page
select mapping.
Soft reset value
: 0x07
WP
Write-protects user Flash banks (not the PC BIOS Boot block). Possible
values include:
0
Write protects user Flash banks.
1
The user may update any of the Flash user banks.
BC2:0
Selects one of these Flash banks:
Figure 4-3. BIOS paging
PC
BIOS,
1M
512K
User
512K
User
512K
User
512K
User
512K
User
512K
User
512K
User
111
110
101
100
011
010
001
000
FFFF,FFFF
FFF0,0000
BC[2:0]
Strata flash
Boot Block
Flash
FFF8,0000
Table 4-18. BIOS control register
R/W
Index
Default
D7
D6
D5
D4
D3
D2
D1
D0
R/W
0x08
0x07
~WP
—
—
—
—
BC2
BC1
BC0
Table 4-19. Flash banks
BC2
BC1
BC0
Page
1
1
1
7
PC BIOS (POR default)
1
1
0
6
512k user space
1
0
1
5
512k user space
1
0
0
4
512k user space
0
1
1
3
512k user space
0
1
0
2
512k user space
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