The W78C438C is a high-performance single-chip CMOS 8-bit microcontroller that is a derivative of the W78C58 microcontroller family. The W78C438C is functionally compatible with the W78C32, except that it provides either a 64 KB program/1 MB data memory address or memory-mapped chip select logic, five general I/O ports, and four external interrupts. In the W78C32, two I/O ports, Port 1 and Port 3, are available for general-purpose use (Port 3 also supports alternative functions), and Port 2 and Port 0 are used as the address bus and data bus, respectively. To enable Port 0 and Port 2 to also be used as general purpose I/O ports, the W78C438C provides two dedicated address ports (AP5 and AP6) that serve as address output for 64 KB of memory and one address/data port (DP4) that serves as ROM code input and external RAM data input/output. Unlike the W78C32, this product does not require an external latch device for multiplexing low byte addresses. The W78C438C also provides four pins (AP7.0-AP7.3) to support either 64 KB program/1 MB data memory space or memory-mapped chip select logic, one parallel I/O port (Port 8) without bit addressing mode, and two additional external interrupts ( INT2 , INT3 ) . The W78C438C is programmed in a manner fully compatible with that used to program the W78C32, except that the external data RAM is accessed by the "MOVX @Ri" instruction. Address paging is performed by loading page addresses into the HB (high byte) register, which is not a standard register in the W78C32, before execution of the "MOVX @Ri" instruction.