Specifications
Chapter 5: Programming the VMEbus Interface
49
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 8102h. 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-8A is a
function of the address supplied.
When accessing the VMEbus in this manner, the source of the VMEbus address lines is
defined below.
A32
A24
A16
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 must coordinate their use of
the E-page mapping registers.
When using the EPC-8A this way to perform VMEbus accesses, you would typically set
up the E-page window for interrupt acknowledge accesses. Also note that the direct access
mappings do not cover the entire VMEbus A32 address range and do not provide all
VMEbus-defined address modifier encodings, but you can use the E-page mechanism if
needed to provide these.
Generating IACKs in Protected Mode
As noted in the description of protected mode direct VMEbus accesses, the values of
address bits 24 and 25 determine the access mode. Setting bit 26 causes an IACK (if the
Table 5-2. VME access type
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) Llittle 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
31 30 29 0
00 From 486 address bits 29–0
23 0
From 486 address bits 23–0
15 0
From 486 address bits 15–0