Inside the Macintosh: the memory.

Let's start from this post with a series of posts with technical "dissection" of the different areas of the Macintosh mainboard. It's very important to understand how the hardware works.

Of course, a software emulator works differently from the real hardware, and many times it's a question of implementing tricks, especially when the emulator must run on a limited system such as a microcontroller.

So I will take my time in studying components, I will read the datasheets, and I will include also the link to download them.

I usually consider the memory of the computer first. ROM and RAM are absolutely necessary so let's take a look at the motherboard.

The amount of RAM present in the Macintosh 128K is... 128KB!!! WOW, what a surprise. The amount of ROM in total is 64KB. The ROM is made of two 32KB modules, together they make a 16-bit 32K words memory bank.

The geometry of the RAM instead is different. There are sixteen modules of 64K bits of dynamic RAM. These modules store a matrix of one column by 64K rows of single bits. Eight of these modules are addressed together to store a single byte, one bit for each module. Like the ROM there are two blocks of eight modules, that together make a 16-bit 64K words memory bank.

It's easy to understand how this RAM works. Take a look at the pinout of this module. 

 

Basically, the access to the internal storage happens in two main steps, keeping the line "WriteEnable" low for writing one bit, or high for reading one bit. These steps are needed to set the address of the bit in the range 16-bit wide. So RAS and CAS are asserted low to latch the high and low bytes of the address. According to the state of the WE line a bit is put on data pin Q (read), or the bit on data pin D (write) is written in the memory.

This access mechanism is not needed in a software emulator of course, but I am here to understand how this Mac works. But if you are interested in going deeper with access timings, here's the DRAM MT4264 datasheet.

In my emulator, this kind  of memory can be implemented easily with arrays, I think something like:  

uint16_t ram[65536];

const uint16_t rom[32768] = {

   /* Dump of the ROM */

};

But I will be back on the implementation later.