Hi. On 2024-03-08 12:24, Antonio Carlini wrote:
But after sending that I read a bit further in the Hardware Handbook and found:"The 28-bit SBI longword address field allows access to up to 512 Mbytes of main memory. The hardware supports a maximum of 64 Mbytes of main memory. Physical memory operations are performed when bit 27 of the SBI addresses is zero; 1/0 operations occur when bit 27 is one. So SBI could do 256MB of which half was for I/O."
Actually, the SBI have a 1G address space. Yes, there are only 28 address bit, but SBI is always addressing by longwords, so the two low bits are not included, and are implicitly always 0. Your quote actually says exactly this. 28-bit longword address. Up to 512Mbyte of main memory.
But even looking in the 1977 VAX Hardware Handbook (page 13), it's clear that memory space is 512M and I/O space is 512M on SBI. However, it also points out that at that time, the maximum possible memory was 8M, because that was the most you could get into the memory boxes (the limit was the size of individual memory chips). That obviously grew over time...
So there seems to be something that caps physical memory at 64MB (plus perhaps a bit for multiport memory, but that was always just a handful of MB).
Well. Any memory box is just responding on the SBI, so that's where the real hard limit is. Actual memory boxes that contained more than 64M probably never got made. But that's more a limit on the actual design of a specific box than any limit in the hardware itself. A different box would allow more memory. SBI is generic enough that it don't really put any futher limitations here.
Johnny
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