The third index register was dedicated to pointing at the current procedure's stack frame on the call/return stack. Other reserved memory locations controlled operand sizes when that size was not constant. The B3500 was similar to the B2500 but with a faster cycle time and more expansion choices. The B2500 had a maximum of 60 K bytes of core memory and a 2 microsecond cycle time. The B3500 had a maximum of 500 K bytes and a 1-microsecond cycle time. B2500/3500 weighed about . Subsequent machine generations The B2500/B3500 machines were followed by B2700/B3700/B4700 in 1972; The B2800/B3800/B4800 in 1976, The B2900/B3900/B4900 in 1980 (which was the first of the range to load its microcode from floppy disk, rather than implementing it as hardware read-only memory) and finally the Unisys V Series machines V340-V560 in 1985-90. Machines prior to the B4800 had no cache memory. Every operand byte or result byte required its own separate main memory cycle, which limited program performance. To compensate for this, the B3700/B4700 generation used semiconductor main memory that was faster but more expensive and power hungry than the DRAM used in competing machines. The unusual use of decimal numbers as memory addresses was initially no problem; it merely involved using 1-in-5 rather than 1-in-8 decoder logic in the core memory's row selects and bank selects. But later machines used standard memory chips that expected binary addresses. Each 1000-byte block of logical memory could be trivially mapped onto a subset of 1024 bytes in a chip with only 2.3% waste. But for denser chips and larger total memories, the entire decimal address had to be crunched into a shorter quasi binary form before sending the address to the chips, and done again for each cache or memory cycle. This conversion logic slowed the machine cycle somewhat. An attempted redesign in 1975 of the address space was called MS-3 for "Medium Systems 3rd Generation", but that project was cancelled. Machines before the B2900 allowed input numbers with 'undigit' values above 9, but arithmetic on this gave unspecified results. This was used as a form of hexadecimal arithmetic within the MCP and also by some application programmers. Later versions discontinued this and instead supported two new opcodes (binary to decimal and decimal to binary) to support addressing the hard drives available after Burroughs's acquisition of Memorex. Cancellation and retirement Unisys cancelled further V series hardware development in