American Megatrends AMI 386SX, Предварительное руководство, Страница: 31 / 52

includes address translation registers, advanced multitasking hardware and
protection mechanism to support operating systems. In addition, it is object
code compatible with 8086 family of microprocessor. The 80386SX has built-in
features to support coprocessors, DMA and interrupts (both maskable and
non-maskable). It has two modes of operation: Real address mode and Protected
virtual address mode.
In real address mode it operates as a fast 8086 with 32-bit extension if
desired. In Protected mode, software can perform a task switch into tasks
designated as virtual 8086 mode tasks. The virtual 8086 tasks can be isolated
and protected from one another by the use of paging and I/O permission bit map.
3.2 System Performance
Clock Speed
The AMI-386SX operates at 16 MHz, which results in a clock cycle time of 62.5
ns. It can run at two different speeds.
The lower speed is equivalent to a clock speed of 8MHz. For convenience, we
shall assume this speed to be 8 MHz throughout the rest of this manual.
The panel-mounted turbo switch determines the initial speed after power-on.
The speed can be switched at any time while the machine is operating.
If installed the 80387SX Numeric processor runs at the processor clock
frequency.
Data Access: Bus Width
The 80386SX microprocessor supports two types of accesses: Memory, and
Input/Output. Each type of access can be 16- or 8-bit wide. The memory and
I/O devices are 16 or 8 bit wide. The AMI-386SX allows any type of access to a
device of any width. If necessary, the hardware will split a 80386SX bus cycle
into a number of (upto 16 / 8 = 2) cycles to allow access to a 16- or 8-bit
device. All the on-board devices in the memory space is organized 16-bit wide.
These include the on-board DRAM and the EPROM containing the BIOS. All the
on-board I/O devices are 8-bit wide, with the exception of the 80387SX math
coprocessor which is a 16-bit device.
The AMI-386SX can support 16- and 8-bit memory and I/O devices on the I/O
slots.
EGA/VGA RAM Shadow
The memory space from address 0C0000 thru 0DFFFF is reserved for I/O slot ROM.
Most often, only the EGA BIOS (0C0000-0C3FFF), which is accessed through a