Rainbow Electronics AT89C51SND1C - Manual (Page 24)

24
AT8xC51SND1C
4109E–8051–06/03
Figure 16. Lower 128 Bytes Internal RAM Organization
Upper 128 Bytes RAM The upper 128 Bytes of RAM are accessible from address 80h to FFh using only indirect
addressing mode.
Expanded RAM The on-chip 2K Bytes of expanded RAM (ERAM) are accessible from address 0000h to
07FFh using indirect addressing mode through MOVX instructions. In this address
range, EXTRAM bit in AUXR register (see Table 29) is used to select the ERAM
(default) or the XRAM. As shown in Figure 15 when EXTRAM = 0, the ERAM is selected
and when EXTRAM = 1, the XRAM is selected (see Section “External Space”).
The ERAM memory can be resized using XRS1:0 bits in AUXR register to dynamically
increase external access to the XRAM space. Table 26 details the selected ERAM size
and address range.
Table 26. ERAM Size Selection
Note: Lower 128 Bytes RAM, Upper 128 Bytes RAM, and expanded RAM are made of volatile
memory cells. This means that the RAM content is indeterminate after power-up and
must then be initialized properly.
Bit-Addressable Space
4 Banks of
8 Registers
R0-R7
30h
7Fh
(Bit Addresses 0-7Fh)
20h
2Fh
18h
1Fh
10h
17h
08h
0Fh
00h
07h
XRS1 XRS0 ERAM Size Address
0 0 256 Bytes 0 to 00FFh
0 1 512 Bytes 0 to 01FFh
1 0 1K Byte 0 to 03FFh
1 1 2K Bytes 0 to 07FFh