Specifications

EPC-8 Hardware Reference
5 5
5-10
Protected-Mode Direct VMEbus Accesses
Addresses above 256 MB map directly onto the VMEbus. When direct "protected-
mode" addressing of A24 or A16 space, the high-order nibble is used to define the
access mode and byte ordering. For A32 space, the high-order 2 bits define the access
mode leaving 30 bits available for addressing. Thus, only the first 1 Gigabyte of
VMEbus A32 space is directly addressable. All A24 and A16 space is directly
addressable. The chart following shows how this direct mapping is used.
VME access is enabled by two bits in register 8102. Bit 1, if set, enables the E-page
access mechanism, and bit 0, if set, enables the 32-bit access mechanism. If the
E-page mechanism is disabled, then the E-page is available for EXMbus options that
require more DOS memory below 1MB. The type of VME access performed by the
EPC-8 is a function of the address supplied.
Address Range Access Mode & Access Modifiers Byte Order
1xxx0000
- 1xxxFFFF
A16M, AM = 1,0,1,~A25,A24,~A24
little endian
2x000000 - 3xFFFFFF
A24M, AM = 1,1,1,~A25,A24,~A24
little endian
40000000 - 7FFFFFFF
A32M-Supervisor (AM = 001101)
little endian
80000000 - BFFFFFFF
A32M-Supervisor (AM = 001101)
big endian
Cxxx0000
DxxxFFFF
-
or
-
CxxxFFFF
DxxxFFFF
A16M, AM = 1,0,1,~A25,A24,~A24
big endian
Ex000000 - ExFFFFFF
A24M, AM = 1,1,1,~A25,A24,~A24
big endian
When accessing the VMEbus in this manner, the source of the VMEbus address lines
is defined below.
A32
31
30 29 0
00 From 486 address bits 29-0
A24
23 0
From 486 address bits 23-0
A16
15 0
From 486 address bits 15-0
The main purpose of the direct VMEbus access mechanism, as opposed to the
E-page mechanism, is for multitasking 32-bit operating-system environments, where
multiple tasks need to make VMEbus accesses. Without this, the tasks would have to
coordinate their use of the E-page mapping registers.
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