onboard chips: a 3.5 MHz Z80A 8-bit microprocessor from NEC, an uncommitted logic array (ULA) chip from Ferranti, and an 8 KB ROM providing a simple Sinclair BASIC interpreter. The entire machine weighs just . Early versions of the external RAM cartridge contain 15 KB of memory using an assortment of memory chips, while later versions contain 16 KB chips, but the lowest addressed kilobyte is disabled. The front part of the case is occupied by an integrated 40-key membrane keyboard displaying 20 graphic and 54 inverse video characters. Each key has up to five functions, accessed via the SHIFT and FUNCTION keys or depending on context. For example, the P key combines the letter P, the " character, and the BASIC commands PRINT and TAB. The ZX81 uses a standard QWERTY keyboard layout. The keyboard is mechanically very simple, consisting of 40 pressure-pad switches and 8 diodes under a plastic overlay, connected in a matrix of 8 rows and 5 columns. The ZX81's primary input/output is delivered via four sockets on the left side of the case. The machine uses an ordinary UHF television set to deliver a monochrome picture via a built-in RF modulator. It can display 24 lines of 32 characters each, and by using the selection of 2×2 block character graphics from the machine's character set offers an effective 64 × 44 pixel graphics mode, also directly addressable via BASIC using the PLOT and UNPLOT commands, leaving 2 lines free at the bottom. Two 3.5 mm jacks connect the ZX81 to the EAR (output) and MIC (input) sockets of an audio cassette recorder, enabling data to be saved or loaded. This stores each data bit as a number of pulses followed by an inter-bit silence of 1300 µs. Each pulse is a 150 µs 'high' then a 150 µs 'low'. A '0' bit consists of four pulses while a '1' bit is nine pulses, so the baud rate varies between 400 bit/s for all '0's and 250 bit/s for all '1's. A file with equal amounts of '0's and '1's would be stored at 307 bit/s (38 bytes/sec). This provides a somewhat temperamental storage medium for the machine, which has no built-in storage capabilities. The ZX81 requires 420 mA of power at 7–11 V DC, delivered via a custom 9 V Sinclair DC power supply. The ULA chip, described by the ZX81 manual as the "dogsbody"