Is it possible to expand an Amiga 500 rev 8 (not an Amiga 500 plus) to 1.5 MB of Chip RAM using an A501+ expansion?

It is plug and play or are modifications required?

The board is this

It has a 8375 Agnus, 512KB and OCS Denise, no RTC

Update: I wrongly thought the A501+ had 1MB of RAM. So the question is more like "Is it possible to expand Amiga 500 rev 8 to 1.5 MB with a 1MB trapdoor expansion?"

Update 2: A501+ is listed as 512 KB here and as 1024 KB here

  • Can you post a photo of the motherboard? In any case, you need a 2MB Agnus on the motherboard if you want to see more than 1MB of chip ram. – user180940 May 19 '17 at 15:28
  • Added photo. Yes, it has 2MB Agnus. – Valentino Miazzo May 19 '17 at 19:27
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    The A501+ HAS 1MB of RAM. The A501 has 512KB. Which one do you have? – user180940 May 21 '17 at 18:48
  • I don't have one. Indeed I'm trying to decide what to buy. – Valentino Miazzo May 22 '17 at 7:16
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    If you want more memory, you may prefer installing some real Fast memory. For the A500 this usually meant installing a peripheral on the expansion slot on the left (like a hard drive). This will speed up the system. If you want more chip ram, try asking around for people who can solder the additional 512KB ram on the motherboard to get 1MB chip ram. I very much doubt there's pre-AGA software out there requiring more than 1MB chip ram anyway. – user180940 May 22 '17 at 14:32

Multiple sources on the internet point out that Rev 8a is indeed an Amiga 500 Plus and not a regular Amiga 500. And your picture clearly shows A500+ printed on the board. So I guess it can be said you own an A500 Plus, just with a 2MB Agnus but 512KB of memory. Does it have an on board clock chip and battery, out of curiosity?

The A500Plus service manual ( points out that the A501 and A501Plus use the same PCB, just the A501Plus does not have the clock chip and battery soldered in. It also states that the original A501 can be used both on an A500 (where it will result in the 512KB chip + 512KB slow fast memory configuration) and on an A500Plus (where it will be mapped in chip ram - I assume to give 1.5MB of chip ram, since the rest of the document describes the regular A500Plus with 1MB on board). It does not say anything about what happens with an A501Plus on an 512KB A500Plus.

Additionally, at the end, the list of jumpers in the schematics have a "JP2" jumper that selects between "C0 vs. 08 address map" (0x00C00000 being the slow-fast RAM, 0x00080000 the second Megabyte in chip RAM). Also a JP4 seems to select between 512KB and 1MB on board memory (so a 512KB Amiga 500Plus is indeed a supported configuration except it was never officially released??).

My guess is that hardware-wise, you should be able to mount an A501 Plus in your Rev8a board with 512KB, and that you don't really risk damaging anything. However, again, this is just a guess, so you may want to wait for other answers.

However, you probably still won't get 1.5MB chip RAM because, at least in the leaked Os 3.1+ source code (and there's no reason to believe earlier version to behave differently), Exec checks for Chip ram earlier during boot by starting at location 0 and probing every 16KB for memory and exiting as soon as the test fail, and finally creating one (and only one) node in the free memory list for the (continuous) area of Chip RAM that it found.

So you would have two options here:

  • use some kind of command to add the second MB of chip ram starting at 0x080000 to the free mem list, to get 2 non contiguous chip ram areas (a configuration some applications may not like)

  • or solder the missing 512KB of chip ram on the motherboard, fit the A501Plus in to turn it in a regular 1MB A500Plus + 1MB A501 Plus system.

BTW, if you ever try and somewhat manage to have the hardware drive at least part of the memory of the A501Plus, you can use this program go add the region of memory not recognized by Exec:

Any memory card added to the expansion bay will only show up as slow RAM, by default. To increase the amount of chip RAM you would need to make modifications. To get more than 512KB of chip RAM you need to add the missing RAM to the motherboard, and to get more than 1MB of chip RAM you also need to add the missing 74F(AS)139 at the unpopulated U32 location (this chip is what decodes the upper 1MB of chip ram).

Valid options are as follows:

  1. Leave the motherboard alone -> Max 512KB chip + expansion as slow ram
  2. Add 512KB to the motherboard -> 1MB chip + expansion as slow ram
  3. Add the 512KB and U32 to the motherboard -> more than 1MB chip with expansion RAM as chip ram
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    I believe the last part is wrong. If he adds an A501+ to his A500+ with 512KB on the motherboard, Exec will simply not recognize it. So he will still get a system with 512K, not one with 1MB. – user180940 May 20 '17 at 19:15
  • @user180940 I just realized that and was typing a correction as your comment came in. You are absolutely right, it won't be contiguous with the existing chip RAM. – mnem May 20 '17 at 19:16
  • yep, just saw the edit :( – user180940 May 20 '17 at 19:16
  • @user180940 To be clear, with JP2 set the way it is currently, it will still get 1MB of slow ram with the A501+ in the expansion bay. There's just no way to get more than 512KB of CHIP without adding RAM to the motherboard. – mnem May 20 '17 at 19:19
  • yes. basically, what's preventing him to get 1.5MB of chip ram is not some issue caused by the hardware, but by Exec not recognizing the upper 1MB because it looks only for the continuous region starting from 0x0. See also the end of my answer. – user180940 May 20 '17 at 19:25
up vote 0 down vote accepted

This is what I think is the right answer. Anyway, I would like to have some feedback before selecting it as the right answer.

After looking at the schematics I think these are the possible cases and outcomes:

If I add the a501+ (1MB) to the A500 rev8 (0.5MB without U32) then:

  • _BCASL0 = _BCASL1
  • _BCASH0 = _BCASH1
  • Therefore internal and external banks have CAS selected at the same time.
  • 0x000000 _BRAS0 = _XRAS0 --> DRAM chips U16-19 on the a500 rev8, and U1-4 on the a501+
  • 0x080000 _BRAS1 = _XRAS1 --> DRAM chips U5-8 on the a501+
  • NOTE: CPU address space 0x000000 to 0x07FFFF is mapped to TWO banks at the same time. But this should not be a problem since lines are driven to the same logic levels.

So, 1MB of chipram is available.

If I add the a501+ (1MB) and U32 to the A500 rev8 (0.5MB) then:

  • 0x000000 _BRAS0 = _XRAS0 _BCAS*0 driven --> DRAM chips U16-19 on the a500 rev8
  • 0x080000 _BRAS1 = _XRAS1 _BCAS*0 driven --> nothing
  • 0x100000 _BRAS0 = _XRAS0 _BCAS*1 driven --> DRAM chips U1-4 on the a501+
  • 0x180000 _BRAS1 = _XRAS1 _BCAS*1 driven --> DRAM chips U5-8 on the a501+

So there are 1.5MB of ChipRAM but AmigaOS will not recognize the 1MB at 0x100000 because not contiguous. OS Patch or post-boot command needed.

If I add the a501+ (1MB) and add 0.5MB to the A500 rev8 (without U32) then:

  • 0x000000 _BRAS0 = _XRAS0 _BCAS*0=_BCAS*1 driven --> DRAM chips U16-19 on the a500 rev8 and U1-4 on the a501+
  • 0x080000 _BRAS1 = _XRAS1 _BCAS*0=_BCAS*1 driven --> DRAM chips U20-23 on the a500 rev8 and U5-8 on the a501+
  • 0x100000 _BRAS0 = _XRAS0 _BCAS*1=_BCAS*0 driven --> U16-19 on the a500 rev8 and U1-4 on the a501+
  • 0x180000 _BRAS1 = _XRAS1 _BCAS*1=_BCAS*0 driven --> U20-23 on the a500 rev8 and U5-8 on the a501+

So the a501+ is masked by the internal DRAM, 1MB of ChipRAM.

If I add the a501+ (1MB), U32 and 512KB DRAM to the A500 rev8 (0.5MB) then it is an A500+ with A501+ --> 2MB ChipRAM

  • FWIW, I had an older A500 but it had a fat agnus, so it could address 1MB chip RAM. There was a hack, and I can't remember specifically where but there is a trace on the motherboard you can cut and the 500 will configure the 512KB trapdoor expansion as CHIP RAM. I did not care about not having the trapdoor RAM as FAST because I had (if memory serves) 8MB in the M-Tec hard disk expansion unit. This miggy served me well, right up into the Internet age. – Wonko Aug 24 '17 at 19:37
  • Bless you, YouTube! – Wonko Aug 24 '17 at 20:05
  • I did the same hack on my A500 - modded the motherboard, replaced Agnus with Fatter Agnus and my 512MB fast RAM became CHIP. – Adam Lawrence Feb 2 at 17:13

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