Editing Intel MCS-51 (section)

=== Program memory ===
Program memory (PMEM, though less common in usage than IRAM and XRAM) is up to 64&nbsp;KB of read-only memory, starting at address 0 in a separate address space. It may be on- or off-chip, depending on the particular model of chip being used. Program memory is read-only, though some variants of the 8051 use on-chip flash memory and provide a method of re-programming the memory in-system or in-application.

In addition to code, it is possible to store read-only data such as [[lookup table]]s in program memory, retrieved by the {{code|lang=asm|MOVC A,@A+DPTR}} or {{code|lang=asm|MOVC A,@A+PC}} instructions. The address is computed as the sum of the 8-bit accumulator and a 16-bit register (PC or DPTR).

Special jump and call instructions ({{code|lang=asm|AJMP}} and {{code|lang=asm|ACALL}}) slightly reduce the size of code that accesses local (within the same 2&nbsp;KB) program memory.<ref>ACALL is a 2-byte subroutine calling instruction, it can access locations within the same 2{{nbsp}}KB segment of memory. The absolute memory address is formed by the high 5 bits of the PC and the 11 bits defined by the instruction.</ref>

When code larger than 64&nbsp;KB is required, a common system makes the code bank-switched, with general-purpose I/O selecting the upper address bits. Some 8051 compilers<ref name="keil"/> make provisions to automatically access paged code. In these systems, the interrupt vectors and paging table are placed in the first 32&nbsp;KB of code and are always resident.