Spectrum could maintain the TV display during tape storage and retrieval operations. As well as being able to save programs, the Spectrum could save the contents of arrays, the contents of the screen memory, and the contents of any defined range of memory addresses. Sinclair Research models Pre-production designs Rick Dickinson came up with a number of designs for the "ZX82" project before the final ZX Spectrum design. A number of the keyboard legends changed during the design phase including ARC becoming CIRCLE, FORE becoming INK and BACK becoming PAPER. The Spectrum reused a number of design elements of the ZX81: The ROM code for things such as floating point calculations and expression parsing were very similar (with a few obsolete ZX81 routines left in the Spectrum ROM). The simple keyboard decoding and cassette interfaces were nearly identical (although the latter was now programmed to load/save at a higher speed). The central ULA integrated circuit was somewhat similar although it implemented the major enhancement over the ZX81: A (fully) hardware based television raster generator (with colour) that indirectly gave the new machine approximately four times as much processing power as the ZX81, simply due to the Z80 now being released from this video generation task. A bug in the ULA as originally designed meant that the keyboard did not always scan correctly, and was rectified by a "dead cockroach" (a small circuit board mounted upside down next to the CPU) for Issue 1 ZX Spectrums. ZX Spectrum 16K/48K The original ZX Spectrum is remembered for its rubber chiclet keyboard, diminutive size and distinctive rainbow motif. It was originally released on 23 April 1982 with 16 KB of RAM for or with 48 KB for ; these prices were reduced to and respectively in 1983. Owners of the 16 KB model could purchase an internal 32 KB RAM upgrade, which for early "Issue 1" machines consisted of a daughterboard. Later issue machines required the fitting of 8 dynamic RAM chips and a few TTL chips. Users could mail their 16K Spectrums to Sinclair to be upgraded to 48 KB versions. Later revisions contained 64 KB of memory but were configured such that only 48 KB were usable. External 32 KB RAM packs that mounted in the rear expansion slot were available from third parties. Both machines had 16 KB of onboard ROM. An "Issue 1" ZX Spectrum can be distinguished from