The last three bits of command information, bits 3-5, specify which Bus Operation will be performed according to
the following:
3 4 5
Operation
0
0
0
unused
0
0
1
unused
0
1
0
Read Double Word
0
1
1
unused
1
0
0
Write High Byte
1
0
1
Write Low Byte
1
1
0
Write Single Word
1
1
1
Write Double Word
3.1.8 Address Modes
The S-bus provides two address modes: physical and nonphysical. The mode is determined by one of six control
bits placed on the bus during the address phase. The first address mode, the physical mode, enables access to the
system memory (physical address space) using standard memory reference options. The physical addressing
scheme used in MV/7800 is the same as that used in all 32-bit ECLIPSE MV/Family computers. MV17800
supports up to 32 megabytes of physical memory, however, only a maximum of 14 megabytes is available.
The second address mode, the nonphysical mode, enables access to local address space (nonphysical address
space). By specifying nonphysical address space, the VLSI chips use the S-bus to communicate with one another
for programmed I/O and access to other storage within the system. There are also nonphysical (local) accesses
made to the SCP,. TOD clock/calendar, and the DRP. Transfers that do not access physical memory are indicated
by a nonphysical address on the bus.
When a nonphysical space access is indicated, one of eight local address spaces is selected by specified address
bits; the remaining address bits select a word within that space. A unique 32-kilobyte local address space is
allocated to each of the following:
Address Space
Encoding - A/DO-15>
unused
1 0 0 0 0 0 0 0 0 0
unused
0 1 0 0 0 0 0 0 0 0
PIO/DCH
0 0 1 0 0 0 0 0 0 0
unused
000 1 0 0 0 0 0 0
FPU
00 0 0
t00000
console
0000al0000
MCU
000000l000
BMC
0000000loo
Field Engineering
00000000lo
uSEQ
000000000l
TOD clock/calendar
00000loolo
Typically, locations in these address spaces store control and status information.
043-003621
3-6
DGC
CONFIDENTIAL - INTERNAL USE ONLY
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