External Data Memory

External data memory is read/write. Access to external data is slower than access to internal data memory because the external data memory is indirectly accessed through a data pointer register which must be loaded with an address.

Several 8051 devices provide on-chip XRAM space that is accessed with the same instructions as the traditional external data space. This XRAM space is typically enabled via dedicated chip configuration SFR registers and overlaps the external memory space.

There may be up to 64 KBytes of external data memory; though, this address 3 space does not necessarily have to be used as memory. Your hardware design

may map peripheral devices into the memory space. If this is the case, your program would access external data memory to program and control the peripheral. This technique is referred to as memory-mapped I/O.

The Cx51 Compiler offers two different memory types that access external data: xdata and pdata.

The xdata memory specifier refers to any location in the 64 KByte address space of external data memory.

The pdata memory type specifier refers to only one (1) page or 256 bytes of external data memory. See “Compact Model” on page 95 for more information on pdata.

Far Memory

Far memory refers to the extended addess space of many new 8051 variants. The Cx51 Compiler uses generic 3-byte pointers to access extended memory spaces. Two Cx51 memory types, far and const far, access variables in extended RAM space and constants in extended ROM space.

The Philips 51MX Architecture provides hardware support for 8MB code and xdata space using universal pointers. The new instructions of the 80C51MX architecture are used by the Cx51 compiler to access far and const far variables.

The Dallas 390 Architecture supports an extended code and xdata address

space in contigious mode with a 24-bit DPTR register and the traditional MOVX 3 and MOVC instructions. Variables defined with far and const far are located in

these extended xdata and code address spaces.

Classic 8051 devices may also use far and const far variables if you configure XBANKING.A51 for your target hardware. This is useful for devices that provide an address extension SFR or additional memory spaces that can be mapped into the xdata space. You may also use xdata banking hardware to extend the address space of a classic 8051 device. Refer to “XBANKING.A51” on page 154 for more information.

NOTE

You need to specify the C51 directive OMF2 and the extended LX51 linker/locater to use the far and far const memory types.

Special Function Register Memory

The 8051 provides 128 bytes of memory for Special Function Registers (SFRs). SFRs are bit, byte, or word-sized registers that are used to control timers, counters, serial I/O, port I/O, and peripherals. Refer to “Special Function Registers” on page 101 for more information on SFRs.