The Commodore 64 has a similar "feature". It has a separate 4-bit per address * 1k "nibble" ram chip for the screen color attributes. The processor and data bus are 8 bits wide. When the processor reads from color ram the low 4 bits come from the color ram chip but the hi 4 bits are disconnected. The returned results look a bit random but are actually whatever data was put on the bus the previous clock cycle.