Task: Question Generation
Input (English): The Macintosh's minimal memory became apparent, even compared with other personal computers in 1984, and could not be expanded easily. It also lacked a hard disk drive or the means to easily attach one. Many small companies sprang up to address the memory issue. Suggestions revolved around either upgrading the memory to 512 KB or removing the computer's 16 memory chips and replacing them with larger-capacity chips, a tedious and difficult operation. In October 1984, Apple introduced the Macintosh 512K, with quadruple the memory of the original, at a price of US$3,195. It also offered an upgrade for 128k Macs that involved replacing the logic board.
Output (English): What was replaced in the upgrade that was offered for 128k Macs?