user: Question: what is the cpu for the calculator with 28 KB of RAM and display size 128×64 pixels 21×8 characters?
Columns: ['Calculator', 'CPU', 'RAM', 'Display Size', 'Programming Languages', 'Link Abilities', 'Year Released', 'Predecessors', 'Successors']
Types: ['text', 'text', 'text', 'text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['TI-73/TI-73 Explorer', 'Zilog Z80 @ 6 MHz', '25 KB of RAM, 512KB of Flash ROM', '96×64 pixels 16×8 characters', 'TI-BASIC , Assembly (via Mallard), Flash Apps', 'I/O Port', '1998/2003', 'TI-80', 'None']
SQL:

assistant: SELECT "CPU" FROM "Table" WHERE "RAM" = '28 KB of RAM' AND "Display Size" = '128×64 pixels 21×8 characters';