The central processor had 60-bit words, while the peripheral processors had 12-bit words. CDC used the term "byte" to refer to 12-bit entities used by peripheral processors; characters were 6-bit, and central processor instructions were either 15 bits, or 30 bits with a signed 18-bit address field, the latter allowing for a directly addressable memory space of 128K words of central memory (converted to modern terms, with 8-bit bytes, this is 0.94 MB). The signed nature of the address registers limited an individual program to 128K words. (Later CDC 6000-compatible machines could have 256K or more words of central memory, budget permitting, but individual user programs were still limited to 128K words of CM.) Central processor instructions started on a word boundary when they were the target of a jump statement or subroutine return jump instruction, so no-op instructions were sometimes required to fill out the last 15, 30 or 45 bits of a word. Experienced assembler programmers could fine-tune their programs by filling these "no-op" spaces with misc instructions that would be needed later in the program.