CHAPTER 11. SYSTEM REGISTERS
There are two system registers used to control and monitor a variety of functions on the ZT 8905.
Each register is read/write capable. Normally, only the system BIOS will use the registers, but they
are documented for application use as needed. Care must be taken when modifying the contents of
these registers as the system BIOS may be relying on the state of the bits under its control.
PROGRAMMABLE REGISTERS
The following topics illustrate the two System registers.
System Register 0
7
6
5
4
3
2
1
0
RSV RSV BUS LED VID
FWP
Register:
Address:
Access:
System Register 0
78h
Read and Write
Flash Memory A20
ZT8905F11REG1
System Register 0
A21
A20
0 = Lower Megabyte
1 = Upper Megabyte
Flash Memory A21 (4MB Only)
0 = Lower Device
1 = Upper Device
Flash Write Protect
0 = Read Only
1 = Read/Write
Video Mux Control
0 = Don't drive cable
1 = Drive cable
LED Drive
0 = LED OFF
1 = LED ON
Bus Owner for BUSRQ*
0 = Service BUSRQ*
1 = Don't Service BUSRQ*
Reserved
Write = 0
Read = Indeterminate
System Register 0
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