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Retrochallenge 2018/09, 2019/03

Setup Environment; Design Memory Management

First week, March 2019

Since the last retrochallenge, I built a new Windows computer, and I had not yet set up any AVR workflow on it. So I spent some time getting Atmel Studio installed, along with a bootloader I like to use... Chip45. I prefer RAW AVR programming to using Arduino. I really like usign AVRDude when I'm using blank new chips. But, if I can spare the flash space for it, I will put on Chip45, because then I can reprogram over the serial port, instead of ISP.

I was able to build the previous retrochallenge's KittyOS code, and flash it with Chip45. To make sure all the files I needed were really in github, I set up the new environment using only files I uploaded there.

I might not do much more this week, but I should at least write up how I imagine memory management to work on this computer

Internal SRAM

Avr-gcc's clib provides a simple malloc. I intend to NOT use that at all, and instead write my own. For a previous AVR project I was goofing around with, I wrote a basic allocator, and I intend to use parts of that here. (Or perhaps rewrite it from scratch.) The Atmega 644 contains 4KB of SRAM. One possible upgrade path for this computer is moving at an Atmega1284, which has 16KB of SRAM. I want any solution here to be able to address up to 16KB of internal SRAM. I am also only going to allow allocations up to 512 bytes. The intention is the SRAM will be used more as a cache for programs running in the VM interpreter, and less for actual data storage.

/* Each block of memory allocated begins with a 2-byte header.  
   This header supports tracking free blocks and reference

#define MASK_FLAGS	0xC000
#define FLAG_FREE	0x8000  /* 1000 0000 0000 0000 */
#define FLAG_HANDLE	0x4000  /* 0100 0000 0000 0000 */
#define MASK_SIZEW	0x01FF  /* 0000 0001 1111 1111 */
#define MASK_REF	0x3E00  /* 0011 1110 0000 0000 */
#define REF_INC		0x0200  /* 0000 0010 0000 0000 */

typedef u16 mheader_t;

There is also the above-mentioned 'handle.' Some regions of memory will be associated with a 'handle'. If the handle flag is set, the 1st word of the block following the header will be handle number.

External SRAM (XRAM)

The CAT-644 has 128K of external SRAM. 64K of this SRAM is devoted to graphics. It is possible to come up with video modes that take less memory (If you don't want wrap-around scrolling, the display only takes 60K of memory), but for now there is no need to work out schemes to expand memory usage. The other 64K will be used for data. The external memory is not directly usable by th CPU, but must be bitbanged in software. Experiments writing to memory shows this is not that slow, but is slower than internal SRAM. XRAM will be used for the bulk of data (and code) storage for the CAT-644

The plan for XRAM is that each block of XRAM will be associated with a handle. Allocating a block of XRAM does not return a direct pointer to it, but returns a handle. For now the implementation of the handle WILL simply be the address in XRAM... so it is a pointer. But it is not a pointer that can be used directly by the AVR... the contents of XRAM need to be read or written programatically. This is where the handles come in. A program needing to read or write a structure pointed to by a handle, will have to call a function. If that structure is already on the SRAM heap, the reference count will be increased. If it is not, then the object will be retrieved from XRAM and put into SRAM.

These are the planned XRAM functions

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