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The 8051 architecture provides many functions (central processing unit (CPU), random-access memory (RAM), read-only memory (ROM), input/output (I/O) ports, serial port, interrupt control, timers) in one package:
The output drivers of Ports 0 and 2, and the input buffers of Port 0, are used in accesses to external memory. In this application, Port 0 outputs the low byte of the external memory address, time-multiplexed with the byte being written or read. Port 2 outputs the high byte of the external memory address when the address is 16 bits wide.
All port input and output can therefore be performed by memory move operations on specified addresses in the SFR region. Also, different status registers are mapped into the SFR, for use in checking the status of the 8051, and changing some operational parameters of the 8051.
Memory-mapped I/O is preferred in IA-32 and x86-64 based architectures because the instructions that perform port-based I/O are limited to one register: EAX, AX, and AL are the only registers that data can be moved into or out of, and either a byte-sized immediate value in the instruction or a value in register DX determines which port is the source or destination port of the transfer.
The SDK-51 uses the external ROM version of the 8051 (8031). It provides a serial port which can support either RS232 or current loop configurations, and also an audio cassette interface to save and load programs. Unlike some of Intel's other SDKs (e.g. SDK-85, SDK-86), the built-in monitor can only be controlled via the built-in QWERTY ...
PORTx: Output port register. Sets the output value on pins configured as outputs. Enables or disables the pull-up resistor on pins configured as inputs. PINx: Input register, used to read an input signal. On some devices, this register can be used for pin toggling: writing a logic one to a PINx bit toggles the corresponding bit in PORTx ...
discrete input and output bits, allowing control or detection of the logic state of an individual package pin; serial input/output such as serial ports ; other serial communications interfaces like I²C, Serial Peripheral Interface and Controller Area Network for system interconnect
Port A has input capture, output compare, pulse accumulator, and other timer functions; port D has serial I/O, and port E has an analog-to-digital converter (ADC).