for the growing laptop computer market of the early 1980s. Intel produced a CMOS 8085 (80C85) used in battery-powered portable computers, such as the Kyocera-designed laptop from April 1983, also sold by Tandy (as TRS-80 Model 100), Olivetti, and NEC. In following years, however, CMOS versions of the Z80 (from both Zilog and Japanese manufacturers) would dominate this market as well, in products such as the Amstrad NC100, Cambridge Z88 and Tandy's own WP-2. Perhaps a key to the initial success of the Z80 was the built-in DRAM refresh, at least in markets such as CP/M and other office and home computers. (Most Z80 embedded systems use static RAM that do not need refresh.) It may also have been its minimalistic two-level interrupt system, or conversely, its general multi-level daisy-chain interrupt system useful in servicing multiple Z80 IO chips. These features allowed systems to be built with less support hardware and simpler circuit board layouts. However, others claim that its popularity was due to the duplicated registers that allowed fast context switches or more efficient processing of things like floating-point math compared to 8-bit CPUs with fewer registers. (The Z80 can keep several such numbers internally, using HL'HL, DE'DE and BC'BC as 32-bits registers, avoiding having to access them from slower RAM during computation.) For the original NMOS design, the specified upper clock-frequency limit increased successively from the introductory 2.5 MHz, via the well known 4 MHz (Z80A), up to 6 (Z80B) and 8 MHz (Z80H). The NMOS version has been produced as a 10 MHz part since the late 1980s. CMOS versions were developed with specified upper frequency limits ranging from 4 MHz up to 20 MHz for the version sold today. The CMOS versions allowed low-power standby with internal state retained, having no lower frequency limit. The fully compatible derivatives HD64180/Z180 and eZ80 are currently specified for up to 33 and 50 MHz respectively. Design Programming model and register set The programming model and register set of the Z80 are fairly conventional, ultimately based on the register structure of the Datapoint 2200. The Z80 was designed as an extension of the Intel 8080, created by the same engineers, which in turn was an extension of the 8008. The 8008 was basically a PMOS implementation of the TTL-based CPU of the Datapoint 2200. The 2200 design allowed 8-bit registers H and L (High and Low) to be paired into