TI486 Microprocessor - Al Kossow's Bitsavers

Address Spaces
Figure 2-20. Offset Address Calculation
Index
Base
Displacement
Offset Address
(Effective Address)
Table 2-17. Memory Addressing Modes
ADDRESSING
SCALE DISPLACEMENT OFFSET ADDRESS (OA)
BASE INDEX
MODE FACTOR (SF) (DP) CALCULATION
Direct X OA=DP
Register indirect X OA=[BASE]
Based X X OA = [BASE] + DP
Index X X OA = [INDEX] + DP
Scaled index X X X OA = ([INDEX] * SF) + DP
Based index X X OA = [BASE] + [INDEX]
Based scaled X X X OA = [BASE] + ([INDEX] * SF)
index
Based index with X X X OA = [BASE] + [INDEX] + DP
displacement
Based scaled X X X X OA = [BASE] + ([INDEX] * SF) + DP
index with
displacement
2.4.2.2 Real Mode Memory Addressing
In real mode operation, the TI486 addresses only the lowest 1 MByte (2 20 ) of
memory. To calculate a physical memory address, the 16-bit segment base
address located in the selected segment register is shifted left by four bits and
then the 16-bit offset address is added. For the TI486SLC/E, the resulting
20-bit address is then extended with four zeros in the upper address bits to
create the 24-bit physical address. For the TI486DLC/E, the resulting 20-bit
address is then extended with 12 zeros in the upper address bits to create the
32-bit physical address. Figure 2-21 illustrates the real mode address
calculation. Physical addresses beyond 1 MByte cause a segment limit
overrun exception.
The addition of the base address and the offset address may result in a carry.
Therefore, the resulting address may actually contain up to 21 significant
address bits that address memory in the first 64 KBytes above 1 MByte.
2-40
Programming Interface