Geneve logical memory map

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The logical address map determines which devices or memory areas appear in the logical address space. These are the addresses as seen from the CPU or the program that is run, but they need not correspond to the same physical address at all times. The correspondence is provided by the mapper.

The memory management in Native mode is supported by a XOP call.

Native mode

The native mode is also called MDOS mode. It is the mode which the Geneve initially boots to, typically with white letters on dark blue, and 80 column display.

Memory map

0000 Reserved Operating system
0400 Free space Onboard memory or expansion
F000 On-chip memory 252 bytes Hides mapped page up to F0FB (see below)
F100 Video ports See below
F110 Mapper See link
F118 Keyboard Single address F118 with 8 mirrors
F120 Sound chip Single address F120 with 8 mirrors on even addresses
F130 Clock chip
F140 Free space Onboard memory or expansion
FFFA Decrementer
FFFC NMI vector

On-chip memory

The on-chip memory of the TMS9995 consists of the address range F000-F0FB and FFFC-FFFF, 256 bytes. The latter four bytes are used for the NMI vector (formerly known as LOAD interrupt vector). The word at FFFA is the decrementer; it may be used as a memory storage when the decrementer is halted.

The memory area is equipped with SRAM at 0 wait states, with full 16 bit access. It is the fastest RAM in the system. Hence, register workspaces should make good use of this area. In that respect, it resembles the Scratch pad RAM of the TI-99/4A.

The on-chip RAM cannot be mapped in or out; it is always present at the specified logical addresses. According to the 9995 specifications, on-chip memory accesses are visible outside; the external address bus is set (only for the even address, as these are full 16-bit accesses), and unspecified data is output on the external data bus on write operations. This means that the hidden RAM is overwritten.

Video ports

Decoding: 1111 0001 0000 xpp0 (pp = port number; x = don't care)

Addresses are mirrored at position+8 (F100 = F108,...)

Read Write
F100 (Port 0) VRAM VRAM
F102 (Port 1) Status register Address counter or video register
F104 (Port 2) - Palette register
F106 (Port 3) - Indirect video register access

GPL mode

The GPL mode is the compatibility mode, which is provided to run TI-99/4A programs.

Memory map

0000 GPL interpreter Operating system
2000 Free space 8 KiB Onboard memory or expansion
4000 DSR space Normally used by the GeneveOS DSR
6000 Cartridge ROM contents Loaded from disk
8000 Mapper See link
8008 Keyboard
8010 Clock chip
8020 Free space 992 bytes Also includes the "PAD RAM" (16-bit RAM of the TI-99/4A)
8300 Here, its speed depends on SRAM or DRAM mapping
8400 Sound chip
8600 External bus Any device in the PEB that maps to this address
8800 Video ports
9000 Speech synthesizer if plugged into the PEB
9800 GRAM
A000 Free space 20 KiB
F000 On-chip memory Hides mapped page up to F0FB (see above)
F0FC Free space 3838 bytes
FFFA Decrementer
FFFC NMI vector

Video ports

Decoding: 1000 1wxx xxxx xpp0 (pp = port number; x = don't care; w = write)

Addresses are mirrored 64 times (8800 = 8810 = 8820 = ... 8BF0)

Read Write
8800 (Port 0) VRAM -
8C00 (Port 0) - VRAM
8802 (Port 1) Status register -
8C02 (Port 1) - Address counter or video register
8C04 (Port 2) - Palette register
8C06 (Port 3) - Indirect video register access