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| sst-6809:memory_latch_select_256b [2025/05/23 19:35] – [ASSIST09] robert | sst-6809:memory_latch_select_256b [2025/05/23 19:55] (current) – [RAM Mapping Note] robert | ||
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| ====== Memory Latch Select 256b ====== | ====== Memory Latch Select 256b ====== | ||
| - | The paging latch. | + | ^ Address Range ^ Usage ^ |
| + | | 0000-7FFF | ||
| + | | 8000-DFFF | ||
| + | | E000-EFFF | ||
| + | | F000-FFEF | ||
| + | | FFF0-FFFF | ||
| + | |||
| + | ===== The Paging Latch ===== | ||
| ^ ^ ^^^ ^^^^ | ^ ^ ^^^ ^^^^ | ||
| | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 | | | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 | | ||
| | Status LED| EEPROM 4K Page ||| RAM 64K Map |||| | | Status LED| EEPROM 4K Page ||| RAM 64K Map |||| | ||
| - | | 1 = on, 0 = off | 0-7 ||| 0-15 |||| | + | | 1 = off, 0 = on | 0-7 ||| 0-15 |||| |
| ===== Status LED ===== | ===== Status LED ===== | ||
| Line 30: | Line 37: | ||
| E.g. if you write a 1 to EF00 it will duplicate the upper bank in the lower bank. | E.g. if you write a 1 to EF00 it will duplicate the upper bank in the lower bank. | ||
| + | ==== RAM Mapping Note ==== | ||
| + | |||
| + | :!: | ||
| + | |||
| + | Be sure you have RAM switching code in ROM before switching to RAM pages 2 or higher. | ||
| + | |||
| + | ROM code should include functions to: | ||
| + | * Register-only Block Copy from RAM pages 0 and 1 into pages 2 and higher | ||
| + | * The block copy must be split in two halves: | ||
| + | * The first half gets the source byte into a register (only) | ||
| + | * Switch the RAM page page but let the Program Counter fall through into the next ROM instruction which is the start of the second half of the copy | ||
| + | * The second half takes the register and posts it into a destination address of the destination RAM page | ||
| + | * Then switch BACK to the source RAM page and fall through into the next ROM address which continue copying from source RAM page | ||
| + | * Transfer the Program Counter into the destination address of the destination RAM page to start running code from | ||
| ===== Upper 32K ===== | ===== Upper 32K ===== | ||