The HP 40gs was released in 2006 as a CAS-enabled counterpart to the HP 39gs in European markets. The CAS used is based on Erable, the same CAS used in the 48 series, as well as the 49/50 series. In order to accommodate the CAS software, the 40gs has an expanded 2MB memory compared to the 39gs' 1MB. Other than this difference, the hardware specs between the 39gs and the 40gs are identical. 256kB of RAM, and a 75 MHz ARM9 processor.
The calculator's HOME interface is almost identical to the other calculators in the series, even the 38g. The only difference is the "CAS" menu that is accessible by pressing the rightmost function key, and the CAS interface is very similar to that of the 49/50 series' equation writer.
Unlike many of HP's other calculators, the 40gs has a strictly infix input notation, and does not feature a RPN mode.