GeneveOS Memory Management Functions

Memory management in MDOS is available via XOP 7. In the MDOS sources the relevant files are manage2s and manage2t.

Fast RAM is all memory with 0 wait states (SRAM), while Slow RAM is 1 wait state RAM, offered by the DRAM chips. The on-chip RAM is not considered here as it is not manageable by MDOS. The 256 bytes of on-chip RAM are always visible in the logical address area F000-F0FB and FFFC-FFFF. Any page that is mapped to the E000-FFFF area will be overwritten by operations in that area; this must be considered when using the E000-FFFF area.

Two classes of memory management functions are available: XOPs for user tasks, and XOPs exclusively used by the operating system.

User-task XOPs

Available memory pages

Opcode: 0

	Input	Output
R0	Opcode (0000)	Error code (always 0)
R1		Number of free pages
R2		Number of fast free pages
R3		Total number of pages in system

Allocate pages

Opcode: 1

	Input	Output
R0	Opcode (0001)	Error code (0, 1, 7, 8)
R1	Number of pages	Number of pages actually fetched to complete map as required
R2	Local page address	Number of fast pages fetched
R3	Speed flag (if not 0, only fast)

This function is used to claim pages from the set of unassigned physical pages. This is particularly useful when the application requires more than just small buffers. As it was common with TI application programming, buffers were usually placed statically in the program, that is, a portion of memory (like 256 bytes) was reserved in the program code to be used for storing data. Assembly language offers the BSS and BES directives for this purpose. This is not recommended when the buffer is very big, like 100 KiB. In that cases, dynamic allocation is the typical solution also known from other systems.

Free pages

Opcode: 2

	Input	Output
R0	Opcode (0002)	Error code (0, 2)
R1	Number of pages
R2	Local page address

Error code 2 is returned when the local page address (R2) is 0; page 0 cannot be freed in this way.

Map local page at excution page

Opcode: 3

	Input	Output
R0	Opcode (0003)	Error code (-, 2, 3)
R1	Local page number
R2	Execution page number

The local page numbers are the IDs of the allocated memory pages. The task contains a linked list of pointers to these local pages; the last list element points to 0. The list of pages is not sorted, so the search for a special local page may require to traverse the complete list.

Error code 2 is returned when the local page number or the execution page number is 0.

Error code 3 is returned when the execution page number is higher than 7 or the list does not contain an entry with the given local page number.

Get address map

Opcode: 4

	Input	Output
R0	Opcode (0004)	Error code (-, 8)
R1	Pointer to buffer	Count of pages reported
R2	Buffer size

On return of the call, the memory locations pointed to by R1 contain a sequence of physical page numbers that have been allocated. R1 contains the length of this sequence.

Error code 8 is returned when the buffer is too small to hold the list of numbers of allocated pages.

Declare shared pages

Opcode: 5

	Input	Output
R0	Opcode (0005)	Error code
R1	Number of pages to declare as shared
R2	Local page address
R3	Type to be assigned to shared pages

Release shared pages (type)

Opcode: 6

	Input	Output
R0	Opcode (0006)	Error code
R1	Type

Also have to check their current execution map

Get shared pages

Opcode: 7

	Input	Output
R0	Opcode (0007)	Error code
R1	Type
R2	Local page number for start of shared area

Return size of shared page group

Opcode: 8

	Input	Output
R0	Opcode (0008)	Error code
R1	Type	Number of pages in shared group

Privileged XOPs (only available for operating system)

Release task

Opcode: 9

	Input	Output
R0	Opcode (0009)

Task header at >8000

Page get

Opcode: 10

	Input	Output
R0	Opcode (000A)	Error code
R1	Page number to get (if high byte not 0, get first available)	Pointer to node
R2	Speed flag (if not 0, only fast)	Page number from node

Add page to free pages in system

Opcode: 11

	Input	Output
R0	Opcode (000B)	Error code (if no free nodes available)
R1	Page number	Pointer to node

Add a node to the list of free nodes

Opcode: 12

	Input	Output
R0	Opcode (000C)	Error code (always 0000)
R1	Pointer to node

Link a node to the specified node

Opcode: 13

	Input	Output
R0	Opcode (000D)	Error code (always 0000)
R1	Pointer to node
R2	Pointer to node to link to

Get address map (system)

Opcode: 14

	Input	Output
R0	Opcode (000E)	Count of valid pages

Error codes

Code	Meaning
00	No error
01	not enough free pages
02	can't remap execution page zero
03	no page at LOCAL address
04	user area not large enough for list
05	shared type already defined
06	shared type doesn't exist
07	can't overlay shared and private memory
08	out of table space