External X Memory Bit (Ex)—Bit 3; Saturation (Sa)—Bit 4; Table 5-3 Program Ram Operating Modes; Table 5-4 Mac Unit Outputs With Saturation Mode Enabled (Sa = 1) - Motorola DSP56800 Manual

MB MA Chip Operating Mode
0 0
0 1
1 0
1 1
The MB and MA bit values are typically established on reset from an external input. Once the chip leaves
reset, they can be changed under software control. For more information about how they are configured on
reset, consult the appropriate device's user's manual.
5.1.9.2 External X Memory Bit (EX)—Bit 3
The external X memory (EX) bit (OMR bit 3), when set, forces all primary data memory accesses to be
external. The only exception to this rule is that if a MOVE or bit-field instruction is executed using the I/O
short addressing mode, then the EX bit is ignored, and the access is performed to the on-chip location. The
EX bit allows access to internal X memory with all addressing modes when this bit is cleared. This bit is
cleared by processor reset.
The EX bit is ignored by the second read of a dual-read instruction, which uses the XAB2 and XDB2 buses
and always accesses on-chip X data memory. For instructions with two parallel reads, the second read is
always performed to internal on-chip memory. Refer to Section 6.1, "Introduction to Moves and Parallel
Moves," on page 6-1 for a description of the dual-read instructions.
5.1.9.3 Saturation (SA)—Bit 4
The Saturation (SA) bit enables automatic saturation on 32-bit arithmetic results, providing a user-enabled
Saturation mode for DSP algorithms that do not recognize or cannot take advantage of the extension
accumulator. When the SA bit is set, automatic saturation occurs at the output of the MAC unit for basic
arithmetic operations such as multiplication, addition, and so on. The SA bit is cleared by processor reset.
Automatic limiting as outlined in Section 3.4.1, "Data Limiter," on page 3-26 is not affected by the state of
the SA bit.
Saturation is performed by a dedicated circuit inside the MAC unit. The saturation logic operates by
checking 3 bits of the 36-bit result out of the MAC unit—EXT[3], EXT[0], and MSP[15]. When the SA bit
is set, these 3 bits determine if saturation is performed on the MAC unit's output and whether to saturate to
the maximum positive or negative value, as shown in Table 5-4.
Table 5-4. MAC Unit Outputs With Saturation Mode Enabled (SA = 1)
EXT[3]
0
0
0
0
Table 5-3. Program RAM Operating Modes
Single Chip
(Reserved)
Normal Expanded
Development
EXT[0] MSP[15]
0 0
0 1
1 0
1 1
Program Controller
Architecture and Programming Model
Reset Vector
Internal PROM P:$0000
(Reserved)
External Pmem P:$E000
External Pmem P:$0000
Result Stored in Accumulator
(Unchanged)
$0 7FFF FFFF
$0 7FFF FFFF
$0 7FFF FFFF
Program Memory
Configuration
Internal Pmem enabled
(Reserved)
Internal Pmem enabled
Internal Pmem disabled
5-11