Editing Intel MCS-51 (section)

=== Internal RAM ===
[[Internal RAM]] (IRAM) has an 8-bit address space, using addresses 0 through 0xFF. IRAM from 0x00 to 0x7F contains 128 directly addressable 1-byte registers, which can be accessed using an 8-bit absolute address that is part of the instruction. Alternatively, IRAM can be accessed indirectly: the address is loaded into R0 or R1, and the memory is accessed using the <code>@R0</code> or <code>@R1</code> syntax, or as stack memory through the stack pointer SP, with the <code>PUSH</code>/<code>POP</code> and <code>*CALL</code>/<code>RET</code> operations.

The original 8051 has only 128 bytes of IRAM. The 8052 added IRAM from 0x80 to 0xFF, which can ''only'' be accessed indirectly (e.g. for use as stack space). Most 8051 clones also have a full 256 bytes of IRAM.

Direct accesses to the IRAM addresses 0x80–0xFF are, instead, mapped onto the special function registers (SFR), where the accumulators A, B, carry bit C, and other special registers for control, status, etc., are located.