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NEC PC-6xx1 is a range of Z80-compatible CPU based computers released by NEC in the early 80's.

(infos taken from wikipedia) http://it.wikipedia.org/wiki/NEC_PC-6001

• 6001 : no built in mass data storages
• 6601 : Floppy disk interface and drive

• CPU : PD 780C-1, seems to be Z80 clone with various clocking, according to video system.

• Co-CPU : PD8049

PC-6001

• Year : 1981
• CPU clock : 3,9936 MHz
• RAM :16KB (max 32K)
• ROM : 16KB Basic, 4KB caratteri

• Graphics :
  • 128x192x4(/15 colours)
  • 256x192x2(/15 colours)
  • Text mode : 32x16

• Sound :AY-3-8910
• Built-in Peripherals :
• Basic and firmware :N60(Microsoft 16K)

PC-6001 MK2 and PC 6601

• Year : 1983
• CPU clock  : 4 MHz
• RAM : 64KB or 64KB + 1KB (floppy interface)
• ROM :32KB Basic, 32KB caratteri kanji, 16KB generatore caratteri(modo 4), 16KB sintesi vocale

• Graphics :
  • 160x200x15 (/15 colours)
  • 320x200x4 (/15 colours)
  • Text mode : 40x20 (40x25?)

• Sound :AY-3-8910, speech synthetiser (japanese)
• Built-in Peripherals :6601 : 1 Floppy 1D singolo lato
• Basic and firmware :N60(16 e 32K RAM), N60 Extended(16 e 32K RAM), N60m (64K RAM)

PC-6001 MK2 SR and PC6601 SR

• Year : 1984

• CPU clock  : 3,58 MHz
• RAM : 64KB or 64KB + 1KB (floppy interface)
• ROM :32KB N66SR Basic, 32KB N66 Basic, 32KB caratteri kanji, 16KB generatore caratteri(modo 5), 32KB sintesi vocale

• Graphics :
  • 320x200x15 (/15 colours)
  • 640x200x4 (/15 colours)
  • Text mode : 80x25

• Sound : FM synthesis, speech synthetiser (japanese)
• Built-in Peripherals : 6601 SR : 1 Floppy disk 1DD singolo lato, doppia densità
• Basic and firmware :N60, N60 Extended, N66, N66 SR

Just check the specs from the PC-6001 MK2 and the PC-6601.

Those are basically Amstrad CPCs with extra sound capability and less video modes and palette.