The Mythical Mac 256K!

[Snial]

This MacGUI blog post covers the boot process for the early System Software on a Mac 512K (or Mac 128K). In part of it, he talks about the Mythical 256KB Mac:

At the beginning of the boot blocks are several stored parameters. The version number is two bytes. Another two bytes hold flags for the secondary sound and video pages. Then come a series of seven, 16-byte long file names. These are the names of the System resource file, Finder (or shell), debugger, disassembler, startup screen, initial application to run, and clipboard.

Following the names is a value for how many open files there may be at once, and the event queue size. Then come three, 4-byte values which are system heap size for a Mac with 128K of RAM, 256K, and 512K of RAM, respectively. If you could find me a Mac with 256K of RAM, I'd sure be impressed.

<snip>
If you landed from planet Krypton with a Mac 256K, your system heap size would be $8000 (decimal 32,768).

Disappointingly, as he also covers in this blog post, the 64K ROM only checks for the 128K or 512K Mac models

FindMemSize:
4002DE:                  LEA     $0007FFFC,A0      ; 512K RAM
4002E4:                  LEA     $0001FFFC,A1      ; 128K RAM
4002EA:                  CLR.L   (A0)+
4002EC:                  TST.L   (A1)+
4002EE:                  BNE.S   *+$02             ; branch if 512K
4002F0:                  MOVE.L  A1,A0         ; this is a 128K machine
4002F2:                  MOVE.L  A0,$0108      ; set MemTop

But, I am interested in the Mythical Mac 256K and I was wondering how easy it is to create one using the miniVMac emulator. It turns out it is indeed possible!

Here's the proof:




On a Mac 128K, this 19kB MacWrite 1.0 document of scintillating prose takes up 87% of the remaining RAM (13% free) - it's pretty much the largest document you can write without 'Disk-based' MacWrite. On a Mac 512kB it takes up 5%, leaving 95% free (because 19kB/512kB = 3.7%), but on a Mac 256kB it takes up 23% with 77% free.

Amazing! So, how was it done? It wasn't really that hard. First I had to change a bit of the 64K ROM so that if the Mac didn't have 128kB, it defaulted to 256kB. This involved changing byte 0x2e1 to 0x3 (instead of 0x7 as per the "LEA $0007FFFC,A0 ; 512K RAM" above); then I had to change the ROM checksum so that it would match, which involved changing byte 3 to 0x4c. (The Checksum merely involves adding up all the (32-bit zero-extended) 16-bit words in the ROM and then XORING it with the 32-bit value at the beginning of the ROM at 0x400000).

So, a real Mac 256kB with this ROM will pass the checksum. However it won't run on miniVMac, because it modifies the ROM and then changes the standard ROM checksum to match what it would be. So, I had to modify miniVMac a bit too.

The setup needs to properly support 256kB and I had to change setup/SPBLDOPT.i. I think it already supported a 256K message option.:

enum {
    gbk_msz_128K,
    gbk_msz_256K,
//...etc

But it didn't really process the message properly, so I had to add a bit to:

LOCALFUNC char * GetMemSizName(int i)
{
    char *s;

    switch (i) {
        case gbk_msz_128K:
            s = "128K";
            break;
        case gbk_msz_256K:
            s = "256K";
            break;

to support 256kB as a memory size; and then similarly a bit later:

LOCALFUNC tMyErr ChooseMemBankSizes(void)
{
    tMyErr err;
    RAMa_Size = 0;
    RAMb_Size = 0;

    switch (cur_mdl) {
        case gbk_mdl_Twig43:
        case gbk_mdl_Twiggy:
        case gbk_mdl_128K:
        case gbk_mdl_512Ke:
            if (cur_msz == gbk_msz_128K) {
                RAMa_Size = ln2_msz_128K;
            } else
            if (cur_msz == gbk_msz_256K) {
                RAMa_Size = ln2_msz_256K;
            } else

To support 256K. Then I hacked 2 files in the main source code, because I couldn't be bothered to figure out the options for disabling RAM or ROM checking properly. In CONTROLM.h:

LOCALFUNC tMacErr ROM_IsValid(void)
{
#if 0 //CheckRomCheckSum

and in ROMEMDEV.c I forced DisableRomCheck:

GLOBALFUNC blnr ROM_Init(void)
{
#if 1 // DisableRomCheck

With these changes I was able to setup miniVMac:

./setup_t -t larm -m 128K -mem 256K > setup128K256K.sh
sh ./setup128K256K.sh
make

Then boot up from a 410kB disk containing System 1.1 and MacWrite 1.0 with that document:
./minivmac Blank2.img

 

[Phipli]

But what will you do with all that RAM?

 

[Snial]

It probably seems pretty odd that someone would want to create a 256kB Mac when many GB of RAM are the order of the day and every Mac after the original one had at least twice as much RAM as that. But I do have a reason, which I might explore in a future post. In the meantime though I just wanted to add a bit about why, most likely, the Mac 256kB never existed.

For people well versed in early 80's hardware it's pretty obvious. The underlying reason is that the 68000 had a 16-bit data bus and the RAM chips (DRAM chips in 1984) were only 1-bit wide. Therefore, to create 16-bit wide memory you needed 16 DRAM chips. In 1984, the most common chips were 64kbit DRAMs, so the first Mac had 16 x 1-bit x 64kBit = 1MBit of RAM = 128kB of RAM.

Also, the design of the chips meant that the address bus was multiplexed: address lines A0..A7 were also used for A8..A15 and the interpretation was set by another couple of pins: /RAS (Row Address Select for A8..A15) and /CAS (Column Address Select). So, this meant that when chip density was improved, adding another address line would effectively add two: A0..A7,A8 for /CAS and then A9..A16,A17 for /RAS. A Macintosh using those chips would then have 512kB instead of 128kB (16 x 1-bit x 256kBit = 4MBit of RAM = 256kB of RAM).

Which is indeed why the first upgrade for a Mac 128K was to boost it to 512K. In fact these chips could simply replace the previous set of chips if a solder bridge was also made on the motherboard. There was an article in Personal Computer World (UK) at the time which explained how to do it for the brave Mac DIYer!

Now, it turns out that 1-bit x 256kBit RAM chips weren't the only choices available in the mid to late 1980s, there was also a 4-bit x 64kBit RAM chip. With these chips Apple could have made a Mac 256K, but it would have required a Motherboard redesign; whereas the upgrade to 512K didn't. Hence, no Mac 256K.

UNTIL NOW !

But what will you do with all that RAM?

The sky's the limit mate ;-) ! I can MacWrite 1.0 a document about 148k long for a start!

 

[cheesestraws]

this is very silly, nice one

 

[tcole]

Love seeing this, mostly becuase I worked with both Randy and Ed for quite a few years somewhat recently.

 

[ClassicGuyPhilly]

I'll be sure to give your Macintosh 256k an honorable mention on the Difinitive Guide to Compacts!

 

[Snial]

Love seeing this, mostly because I worked with both Randy and Ed for quite a few years somewhat recently.

Amazing! Was the MacWrite 1.0 source code ever released into the public domain? Is it possible? I know MacPaint was.

https://github.com/samoli/MacPaint

-cheers from Julz

 

[Snial]

I'll be sure to give your Macintosh 256k an honorable mention on the Difinitive Guide to Compacts!

Brilliant! Thanks!

 

[Jockelill]

It probably seems pretty odd that someone would want to create a 256kB Mac when many GB of RAM are the order of the day and every Mac after the original one had at least twice as much RAM as that. But I do have a reason, which I might explore in a future post. In the meantime though I just wanted to add a bit about why, most likely, the Mac 256kB never existed.

For people well versed in early 80's hardware it's pretty obvious. The underlying reason is that the 68000 had a 16-bit data bus and the RAM chips (DRAM chips in 1984) were only 1-bit wide. Therefore, to create 16-bit wide memory you needed 16 DRAM chips. In 1984, the most common chips were 64kbit DRAMs, so the first Mac had 16 x 1-bit x 64kBit = 1MBit of RAM = 128kB of RAM.

Also, the design of the chips meant that the address bus was multiplexed: address lines A0..A7 were also used for A8..A15 and the interpretation was set by another couple of pins: /RAS (Row Address Select for A8..A15) and /CAS (Column Address Select). So, this meant that when chip density was improved, adding another address line would effectively add two: A0..A7,A8 for /CAS and then A9..A16,A17 for /RAS. A Macintosh using those chips would then have 512kB instead of 128kB (16 x 1-bit x 256kBit = 4MBit of RAM = 256kB of RAM).

Which is indeed why the first upgrade for a Mac 128K was to boost it to 512K. In fact these chips could simply replace the previous set of chips if a solder bridge was also made on the motherboard. There was an article in Personal Computer World (UK) at the time which explained how to do it for the brave Mac DIYer!

Now, it turns out that 1-bit x 256kBit RAM chips weren't the only choices available in the mid to late 1980s, there was also a 4-bit x 64kBit RAM chip. With these chips Apple could have made a Mac 256K, but it would have required a Motherboard redesign; whereas the upgrade to 512K didn't. Hence, no Mac 256K.

UNTIL NOW !


The sky's the limit mate ;-) ! I can MacWrite 1.0 a document about 148k long for a start!

Awesome writeup! Now you "just" need to recreate that redesigned logic board