colour from the palette. The SAM Coupé has four display modes: Mode 4 — 256×192, linear framebuffer, 4 bits per pixel (16 colours) = 24 KB Mode 3 — 512×192, linear framebuffer, 2 bits per pixel (4 colours) = 24 KB Mode 2 — 256×192, linear framebuffer, 1 bit per pixel with separate colour attributes for each 8-wide block of pixels = 12 KB Mode 1 — 256×192, non-linear framebuffer, 1 bit per pixel with separate colour attributes for each 8×8 block of pixels = 6.75 KB (arranged to match the display of the ZX Spectrum for backwards compatibility) The 'attribute' modes borrow their design from the ZX Spectrum, where a bitmap is used to select between a paper and ink colour from two groups of eight colours. The group to use is selected by a 'brightness' flag. The colour block can also be set to 'flash' - that is, alternate between the two colours used for paper and ink. All modes use palette-based Colour look-up tables, selecting from a palette of 128 colours. The palette values consist of 2 bits for each of the red, green and blue components as well as an extra bit which increases the intensity of all three components by a half-step (a 'brightness' bit). The ASIC can be configured to generate interrupts when a line on the display is starting to be emitted, allowing video effects to be synchronised with specific display lines with little effort. By default, it will generate an interrupt for every frame. Typically this interrupt is used to double buffer the frame, read the keyboard/mouse state, and output music. The interrupt state can also be polled directly from the ASIC's status register. The Motorola MC1377P RGB to PAL/NTSC video encoder creates a composite video signal from the machine's RGB- and Sync-signals (output by the ASIC) for the RF modulator. The non-standard SCART display connector includes both composite and RGB output, as well as signals to drive a 16-colour TTL monitor. Access to internal RAM was shared between the display and the CPU, with CPU accesses incurring a speed penalty (memory contention) as it was forced to waited for isochronous ASIC memory-accesses to complete. As a result, the SAM Coupé's CPU effectively ran only around 14% faster than the ZX Spectrum CPU, yet was required to do much more work in SAM's high-resolution modes to produce a similar movement on the