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| Both sides previous revisionPrevious revision | |||
| sst-6809:memory_latch_select_256b [2025/05/23 19:51] – [RAM 64K Map Select] robert | sst-6809:memory_latch_select_256b [2025/05/23 19:55] (current) – [RAM Mapping Note] robert | ||
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| ==== RAM Mapping Note ==== | ==== RAM Mapping Note ==== | ||
| - | :!: Be sure you have RAM switching code in ROM before switching to RAM pages 2 or higher. | + | :!: |
| + | |||
| + | Be sure you have RAM switching code in ROM before switching to RAM pages 2 or higher. | ||
| ROM code should include functions to: | ROM code should include functions to: | ||
| - | * Block copy code and data from RAM pages 0 and 1 into pages 2 and higher | + | * Register-only |
| * The block copy must be split in two halves: | * The block copy must be split in two halves: | ||
| - | * The first half gets the source byte and prepares to hand it to ... | + | * The first half gets the source byte into a register (only) |
| - | * The second half accessible from the destination RAM page | + | * 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 |
| - | * Then transfer | + | * The second half takes the register and posts it into a destination address of the destination RAM page |
| + | * Then switch | ||
| * Transfer the Program Counter into the destination address of the destination RAM page to start running code from | * Transfer the Program Counter into the destination address of the destination RAM page to start running code from | ||
| ===== Upper 32K ===== | ===== Upper 32K ===== | ||