4 ms·

It's pretty different compared to the Amiga. The Amiga flipped the bus between CPU and chipset every cycle and the CPU could only access chip RAM every 1/2 cycl

It's pretty different compared to the Amiga. The Amiga flipped the bus between CPU and chipset every cycle and the CPU could only access chip RAM every 1/2 cycles. No matter how little the chipset was doing, the CPU could only access RAM half the time. The Xbox/PS5 SoCs have both the CPU and GPU behind the memory controller so even under load the CPU will still have access to its full amount of memory bandwidth.

snvzz 3y ago
>No matter how little the chipset was doing, the CPU could only access RAM half the time. Only applicable to CHIP RAM. CPU could access FAST RAM, the default allocation target, all the time. It also was not as simple as odd/even, although there's sense to the simplification. 68000's design meant it could ultimately only access RAM every other cycle, thus it was sensible to aim for that as the default. Higher 68000 models did not have that restriction. And even the original chipset also wasn't limited to half the cycles. This was only true of the default video mode (4 color "hires", 640 pixels wide). The blitter also had a flag (BITHOG) that allowed it to use full DMA, stealing cycles from the CPU.
- rasz 3y ago
  >Only applicable to CHIP RAM. CPU could access FAST RAM, the default allocation target, all the time. "default" haha. Afaik the only Amiga that ever shipping with fast ram was A4000. It was introduced at $4600 in 1992. In 1993 it was down to $2500, same price ARL was selling Pentium 60MHz system with 8MB ram.
  - snvzz 3y ago
    >"default" haha. Yes, default. AmigaOS AllocMem() will use fast32, then fast, then slow, then chip. Thus software needs to request Chip explicitly when required. This is fortunately well respected by software, thanks to "slow ram" being commonly mounted inside Amiga 500's trapdoor. >Afaik the only Amiga that ever shipping with fast ram was A4000 A3000 already shipped with a 1M+1M CHIP/FAST split in 1991. For the other Amiga, fast ram is a priority expansion. Typically over 2x performance for the A1200's CPU just from having any, via trapdoor expansion slot. On A500, the most sold model, you'd get this through the left expansion slot, either as a standalone board or mounted inside an HDD adapter, back in the day. Today, A500 CPU socket adapters giving 8MB Fast RAM are common.
  - deleted 3y ago
    [deleted]

snvzz 3y ago

>No matter how little the chipset was doing, the CPU could only access RAM half the time. Only applicable to CHIP RAM. CPU could access FAST RAM, the default allocation target, all the time. It also was not as simple as odd/even, although there's sense to the simplification. 68000's design meant it could ultimately only access RAM every other cycle, thus it was sensible to aim for that as the default. Higher 68000 models did not have that restriction. And even the original chipset also wasn't limited to half the cycles. This was only true of the default video mode (4 color "hires", 640 pixels wide). The blitter also had a flag (BITHOG) that allowed it to use full DMA, stealing cycles from the CPU.

rasz 3y ago
>Only applicable to CHIP RAM. CPU could access FAST RAM, the default allocation target, all the time. "default" haha. Afaik the only Amiga that ever shipping with fast ram was A4000. It was introduced at $4600 in 1992. In 1993 it was down to $2500, same price ARL was selling Pentium 60MHz system with 8MB ram.
- snvzz 3y ago
  >"default" haha. Yes, default. AmigaOS AllocMem() will use fast32, then fast, then slow, then chip. Thus software needs to request Chip explicitly when required. This is fortunately well respected by software, thanks to "slow ram" being commonly mounted inside Amiga 500's trapdoor. >Afaik the only Amiga that ever shipping with fast ram was A4000 A3000 already shipped with a 1M+1M CHIP/FAST split in 1991. For the other Amiga, fast ram is a priority expansion. Typically over 2x performance for the A1200's CPU just from having any, via trapdoor expansion slot. On A500, the most sold model, you'd get this through the left expansion slot, either as a standalone board or mounted inside an HDD adapter, back in the day. Today, A500 CPU socket adapters giving 8MB Fast RAM are common.
- deleted 3y ago
  [deleted]