Added memory

memory.md

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+## memory/system map
+
+CPU Addressable
+
+| Address | Usage                            |
+| ------- | -------------------------------- |
+| `$0000` | 32k ram (16k addressable)        |
+| `$1000` | ...                              |
+| `$2000` | ...                              |
+| `$3000` | ...                              |
+| `$4000` | I/0 & Hardware Control Registers |
+| `$5000` | ...                              |
+| `$6000` | 32k vram                         |
+| `$7000` | ...                              |
+| `$8000` | ...                              |
+| `$9000` | ...                              |
+| `$A000` | ...                              |
+| `$B000` | ...                              |
+| `$C000` | ...                              |
+| `$D000` | ...                              |
+| `$E000` | 32k eeprom (8k addressable)      |
+| `$F000` | ...                              |
+
+### hardware control register $4000
+
+| Bit       | 7            | 6            | 5        | 4             | 3                          | 2      | 1      | 0      |
+| --------- | ------------ | ------------ | -------- | ------------- | -------------------------- | ------ | ------ | ------ |
+| **Usage** | ROM Page MSB | ROM Page LSB | RAM Page | 32k Bank Page | Character/Hires Video Mode | Unused | Unused | Unused |
+
+### 6845 crtc register interface at $4200 & $4201
+
+when the system starts or when the video mode is changed, crt timing/character size information has to be sent to the 6845. This is done by writing to address `$4200` which internal register you want to access, then reading from or writing to address `$4201`. for example, this is how you would set the number of characters per row to 80:
+
+```asm
+LDA #0 ;The "Horizontal Total" register is R0
+STA $4200
+LDA #80 ;Set the register value to 80 for 80 characters per row
+STA $4201
+```
+
+### keyboard rows $4300, $4301, $4302, $4303, $4304, $4305
+
+each keyboard row is mapped to a read-only 8-bit register, with each key mapped to a single bit. due to the design of this keyboard, row 5 has 5 unused bits
+
+| Address | Desc  | Bit 7     | Bit 6  | Bit 5  | Bit 4 | Bit 3 | Bit 2 | Bit 1 | Bit 0       |
+| ------- | ----- | --------- | ------ | ------ | ----- | ----- | ----- | ----- | ----------- |
+| `$4300` | Row 0 | `esc`     | `w`    | `e`    | `r`   | `t`   | `u`   | `o`   | `backspace` |
+| `$4301` | Row 1 | `tab`     | `q`    | `s`    | `g`   | `y`   | `i`   | `p`   | `enter`     |
+| `$4302` | Row 2 | `shift`   | `d`    | `v`    | `h`   | `k`   | `"`   | `?`   | `a`         |
+| `$4303` | Row 3 | `control` | `z`    | `f`    | `b`   | `j`   | `l`   | `2`   | `4`         |
+| `$4304` | Row 4 | `alt`     | `x`    | `c`    | `n`   | `m`   | `'`   | `1`   | `3`         |
+| `$4305` | Row 5 | `meta`    | `sp 1` | `sp 2` |       |