Sunday, April 23, 2017

Micro-KIM Tutorial: The Memory Map

Let’s revisit the Micro-KIM memory map, introduced in the third tutorial.

+-----------+
| 2K EPROM  |$1fff
| monitor   |
| program   |$1800
+-----------+
| 6532 RIOT |$17ff
| I/O, timer|
| and RAM   |$1740
+-----------+
| optional  |$173f
| I/O, timer|
| and RAM   |$1400
+-----------+
|           |$13ff
| 5K RAM    |
|           |$0000
+-----------+

Since the default kit (without any expansion) only uses the lower address bits to access 8K, memory repeats itself every 8K. You can verify this by storing and inspecting values in, for instance, addresses $0000 and $2000. Any value stored in one address will show up in the other. Although an interesting factoid, there is no reason to let Micro-KIM programs address anything outside the range $0000-$1fff.

Addresses $0000-$13ff contain 5K free RAM (another interesting factoid: the Micro-KIM actually wastes 3K of its 8K RAM chip to keep compatibility with the original KIM-1). This memory region can be used to store data and code. To verify this, while running the monitor program, use the keyboard to store and inspect values. For example, type AD followed by 0200, DA, and FF and you will see the value at this address change. Type + a few times to change the address. Then, if you later revisit this address with AD 0200, you will still see the value FF stored at this address.

The first 256 bytes of RAM with addresses $0000-$00ff are called the zero page. The 6502 microprocessor supports zero-page addressing to access this part of RAM with more compact and more efficient instructions. For example, both the following two instructions load the contents of address $0000 into register X, but the first uses just 2 bytes rather than 3 bytes for the instruction encoding and executes in 3 rather than 4 cycles.

a6 00 ldx $00
ae 00 00 ldx $0000

This page is typically used by programs to store frequently accessed bookkeeping variables. In fact, the monitor program uses a few bytes stored at addresses $00ef-$00ff for various functions. The next page of RAM with addresses $0100-$01ff is used as stack by the 6502 microprocessor and should be left alone. From address $0200 all the way up to $13ff can be used freely though, which is why most programs start with the following command.

.org $0200

Addresses $1400-$173f are reserved for an optional second RIOT chip, but are otherwise unused in the default kit. You can verify this by trying to store anything in this range. Type AD, 1400, DA, followed by any value. No matter how hard you try, the data field in the LED display remains 00. Addresses $1740-$17ff are used by the 6532 RIOT chip. The functions of this peripheral chip are made available as regular memory addresses, a technique referred to as memory mapping. The 6532 provides 128 bytes of free RAM, two parallel I/O data ports, and programmable timers with interrupt capability. Details of this chip are given in later tutorials. For now, type AD 1744 while running the monitor program to get a surprise: the data field on the LED does not show one value, but cycles through many, as shown below.

Memory Mapped Address in the Micro-KIM.

Lastly, addresses $1800-$1fff are taken by the 2K EPROM that contains the monitor program, discussed earlier. Although you can use the monitor program to inspect its own values in this memory region, you can obviously not change the values by typing DA and a value. For example, after typing AD and 1FFF, the data field shows the value 1C corresponding to the second byte of IRQT, as can be seen in the listing of the monitor program in the appendix of the Setup and User’s Manual of Briel Computers. However, typing DA followed by any value has no effect, since this part addresses immutable ROM.

That's it again for now! Please like, share, or comment if you like this series. Also, note that the series is now available as single PDF on my Micro-KIM webpage.

Sunday, April 16, 2017

Micro-KIM Tutorial: Available as Single PDF

If you were following (and hopefully enjoying) the Micro-KIM tutorial, you may have noticed a rather long silence after the last posting. Unfortunately, my day job and a move plus remodeling claimed most of my spare time. However, I plan to continue the tutorial really soon again!

In the meanwhile, I have made all previous tutorials available as a single PDF on my Micro-KIM website, where you can also find the source code of all examples. Future tutorials will be added to this PDF to keep the collection available as a single file.