s3c2410a Samsung Semiconductor, Inc., s3c2410a Datasheet - Page 117

• Manufacturer Part Number: s3c2410a
• Description: 16/32-bit Risc Arm Microprocessor
• Manufacturer: Samsung Semiconductor, Inc.

S3C2410A
OFFSETS AND AUTO-INDEXING
The offset from the base may be either a 8-bit unsigned binary immediate value in the instruction, or a second
register. The 8-bit offset is formed by concatenating bits 11 to 8 and bits 3 to 0 of the instruction word, such that bit
11 becomes the MSB and bit 0 becomes the LSB. The offset may be added to (U = 1) or subtracted from (U = 0) the
base register Rn. The offset modification may be performed either before (pre-indexed, P = 1) or after (post-indexed,
P = 0) the base register is used as the transfer address.
The W bit gives optional auto-increment and decrement addressing modes. The modified base value may be written
back into the base (W = 1), or the old base may be kept (W = 0). In the case of post-indexed addressing, the write
back bit is redundant and is always set to zero, since the old base value can be retained if necessary by setting the
offset to zero. Therefore post-indexed data transfers always write back the modified base.
The Write-back bit should not be set high (W = 1) when post-indexed addressing is selected.
31
Figure 3-17. Halfword and Signed Data Transfer with Immediate Offset and Auto-Indexing
Cond
28
27
000
25
24
P U
23
22
1
[3:0] Immediate Offset (Low Nibble)
[6][5] S H
[11:8] Immediate Offset (High Nibble)
[15:12] Source/Destination Register
[19:16] Base Register
[20] Load/Store
0 = Store to memory
1 = Load from memory
[21] Write-back
0 = No write-back
1 = Write address into base
[23] Up/Down
0 = Down: subtract offset from base
1 = Up: add offset to base
[24] Pre/Post Indexing
0 = Post: add/subtract offset after transfer
1 = Pre: add/subtract offset bofore transfer
[31:28] Condition Field
0 0 = SWP instruction
0 1 = Unsigned halfword
1 1 = Signed byte
1 1 = Signed halfword
21
W
20
L
19
Rn
16
15
Rd
12
11
Offset
8 7 6 5 4 3
1
S H 1
ARM INSTRUCTION SET
Offset
0
3-35