In SX Microcontrollers, SX/B Compiler and SX-Key Tool, mebikerider wrote:

All the 74HCT139 does is decode 2 lines into 4 states; its big brother 74HCT138 is the same except it decodes 3 to 8. For example, set the data bits to 0,1, and set the chip enable line, and the output bit "1" goes low.

They're pretty common on small computer systems; in fact, if you're building a small computer then this would be a prime candidate for selecting between your RAM and ROM.  

In my project, I fed the SX port C to 3 74HCT573 8-bit latches, then when the appropriate 74HCT139 bit was selected, then released, those 8 bits are latched to the outputs of the HCT573's and thus to the RAM address lines.

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