XRAM

Table of Contents

• Banks 0-1: Framebuffer
• Bank 2: Icons
• Holes and Extra RAM

LCD Framebuffer pixel + icon format.

XRAM a special region of memory at the end of the RAM address space which is divided into three banks. Two XRAM banks combine to serve as the framebuffer for the screen, while the third bank drives the 4 icons at the bottom of the screen.

Banks 0-1: Framebuffer

The first two XRAM banks represent the framebuffer. It is stored from top-to-bottom order, with each 48 pixel row being a sequence of 6 bytes. Each pixel is a single bit moving from left-to-right in the order of MSB to LSB. There are two important things to note about its layout:

• It's banked: so the top half of the screen uses bank 0 while the bottom half uses bank 1 of the same 0x180-0x1FB address range.
• It's disjoint: data are only stored consecutively for two rows at a time, then a 4 byte skip occurs in the address range before continuing on to the next two rows.

The following table shows the byte addresses corresponding to each each 1x8 pixel region of the display.

Row/Col	0-7	8-15	16-23	24-31	32-39	40-47
0, 16	180	181	182	183	184	185
1, 17	186	187	188	189	18A	18B
Skip 4 Bytes
2, 18	190	191	192	193	194	195
3, 19	196	197	198	199	19A	19B
Skip 4 Bytes
4, 20	1A0	1A1	1A2	1A3	1A4	1A5
5, 21	1A6	1A7	1A8	1A9	1AA	1AB
Skip 4 Bytes
6, 22	1B0	1B1	1B2	1B3	1B4	1B5
7, 23	1B6	1B7	1B8	1B9	1BA	1BB
Skip 4 Bytes
8, 24	1C0	1C1	1C2	1C3	1C4	1C5
9, 25	1C6	1C7	1C8	1C9	1CA	1CB
Skip 4 Bytes
10, 26	1D0	1D1	1D2	1D3	1D4	1D5
11, 27	1D6	1D7	1D8	1D9	1DA	1DB
Skip 4 Bytes
12, 28	1E0	1E1	1E2	1E3	1E4	1E5
13, 29	1E6	1E7	1E8	1E9	1EA	1EB
Skip 4 Bytes
14, 30	1F0	1F1	1F2	1F3	1F4	1F5
15, 31	1F6	1F7	1F8	1F9	1FA	1FB

Note

Since the top and bottom halves of the screen share the same address range, using different banks, the same address can refer to two different rows, offset by 16.

Bank 2: Icons

Bank 2 of XRAM is used to drive the icons at the bottom of the screen. Again, we have a disjoint layout, where, starting at bit 6 of address 0x181, each icon is assigned two bits lower of the next byte.

	7	6	5	4	3	2	1	0
0x181		FILE
0x182				GAME
0x183						CLOCK
0x184								FLASH

Holes and Extra RAM

The 4-byte skips don't seem to decode RAM, writes are not effective and reads return 0xFF, this pattern persists across all of the framebuffer regions and also in the icon bank. Additionally there are 32 bytes of icon RAM bytes, in which 4 are used to display the icons you normally see and the remaining 28 just being unused RAM in which you can freely write and read from

The icon RAM bytes are distibuted around Bank 2 in this order

Row/Col	0-7	8-15	16-23	24-31	32-39	40-47
0, 16	180	181	182	183	184	185
1, 17	186	187	188	189	18A	18B
Skip 4 Bytes
2, 18	190	191	192	193	194	195
3, 19	196	197	198	199	19A	19B
Skip 4 Bytes
4, 20	1A0	1A1	1A2	1A3
Skip 12 Bytes
6, 22	1B0	1B1	1B2	1B3
remaining is empty Bytes