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Direct Addressing Mode
6-4
6.2 Direct Addressing Mode
In the direct addressing mode, data memory is addressed in blocks of 128
words called data pages. The entire 64K of data memory consists of 512 data
pages labeled 0 through 511, as shown in Figure 6–3. The current data page
is determined by the value in the 9-bit data page pointer (DP) in status register
ST0. For example, if the DP value is 000000000
2
, the current data page is 0.
If the DP value is 000000010
2
, the current data page is 2.
Figure 6–3. Pages of Data Memory
Data Memory
Page 0: 0000h–007Fh
Page 1: 0080h–00FFh
Page 2: 0100h–017Fh
Page 511: FF80h–FFFFh
.
000 0000
OffsetDP value
0000 0000 0
111 1111
0000 0000 0
0000 0000 1
0000 0000 1
1111 1111 1
1111 1111 1
000 0000
111 1111
000 0000
111 1111
0000 0001 0
000 0000
111 1111
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0000 0001 0
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In addition to the data page, the processor must also know the particular word
being referenced on that page. This is determined by a 7-bit offset (see
Figure 6–3). The offset is supplied by the seven least significant bits (LSBs)
of the instruction register, which holds the opcode for the next instruction to be
executed. In direct addressing mode, the content of the instruction register has
the format shown in Figure 6–4.