3 ms·

Can someone explain why ROM was slower? I would have guessed it was faster. Is there some technical limitation? Or was it more of a “can can shadow the ROM so

by MBCook 4y ago

Can someone explain why ROM was slower? I would have guessed it was faster. Is there some technical limitation? Or was it more of a “can can shadow the ROM so save the extra few cents on faster ROM” thing? Just that no one was pushing as hard as for faster RAM.

monocasa 4y ago
It depends on the individual system, and as you expect there are systems where accessing ROM is equivalent to RAM speed wise. You mainly first began to see a speed disparity with full 32 bit systems which had a 32 bit data path to memory. It'd be common to still only have a 16 bit datapath to ROM, necessitating the hardware to split 32 bit transfers in two when accessing ROM. This disparity ended up diverging further as the CPU only had to hit the north bridge (or more recently the memory controller in uncore on the CPU die) but practically having to go all the way out to some of the slowest, legacy interfaces like LPC to hit ROM.
- toast0 4y ago
  The last paragraph is the result of shadow rom working and cost savings. Of course firmware is now in SPI rom, because it's read once and SPI roms cost less than parallel roms and are easier to fit on the motherboard, and performance almost doesn't matter since they're not that big and again, only read once.
justsomehnguy 4y ago
> The other reason is that the Deskpro 386 was almost certainly the first PC where ROM shadowing made a real difference. The Deskpro 386 had 32-bit RAM, significantly faster than 16-bit ROMs. That was not the case with 16-bit PC/AT compatibles with slower CPUs, slower RAM, and a 16-bit data path to both RAM and ROM. On the Deskpro 386, the system RAM was likely several times faster than ROM, and that’s what made ROM shadowing desirable. Also it doesn't make sense to push for a faster ROM if it's wouldn't be utilized or benefit anything.

3 ms·

Can someone explain why ROM was slower? I would have guessed it was faster. Is there some technical limitation? Or was it more of a “can can shadow the ROM so

by MBCook 4y ago

monocasa 4y ago
It depends on the individual system, and as you expect there are systems where accessing ROM is equivalent to RAM speed wise. You mainly first began to see a speed disparity with full 32 bit systems which had a 32 bit data path to memory. It'd be common to still only have a 16 bit datapath to ROM, necessitating the hardware to split 32 bit transfers in two when accessing ROM. This disparity ended up diverging further as the CPU only had to hit the north bridge (or more recently the memory controller in uncore on the CPU die) but practically having to go all the way out to some of the slowest, legacy interfaces like LPC to hit ROM.
- toast0 4y ago
  The last paragraph is the result of shadow rom working and cost savings. Of course firmware is now in SPI rom, because it's read once and SPI roms cost less than parallel roms and are easier to fit on the motherboard, and performance almost doesn't matter since they're not that big and again, only read once.
justsomehnguy 4y ago
> The other reason is that the Deskpro 386 was almost certainly the first PC where ROM shadowing made a real difference. The Deskpro 386 had 32-bit RAM, significantly faster than 16-bit ROMs. That was not the case with 16-bit PC/AT compatibles with slower CPUs, slower RAM, and a 16-bit data path to both RAM and ROM. On the Deskpro 386, the system RAM was likely several times faster than ROM, and that’s what made ROM shadowing desirable. Also it doesn't make sense to push for a faster ROM if it's wouldn't be utilized or benefit anything.