Here's a fragment from a DS89C420 program that I wrote awhile back that used the internal 1kB SRAM:

PMR equ	0C4h	   ;power management register
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orl PMR,#00000001b ;internal 1kB sram enabled as data memory, DMEO (PMR.7) = 1
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movx A,@DPTR       ;read from SRAM (indexed by DPTR, range = 0000h - 03FFh)
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movx @DPTR,A       ;write to SRAM

You can also set the SRAM as a merged program/data memory by setting the PRAME bit in the ROMSIZE sfr. I don't recommend doing so, as running self-modifying code is playing with fire unless you are very, very careful.