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Add memory map diagrams to tasker.md
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Philip Zembrod
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@ -78,7 +78,7 @@ and start the counter again.
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## Implementation
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## Implementation
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As mentioned,
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As mentioned, the multitasker uses cooperative multitasking where
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execution control is yielded via the word PAUSE. PAUSE saves the current state
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execution control is yielded via the word PAUSE. PAUSE saves the current state
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of the currently active task and calls the task switcher. The state of a task
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of the currently active task and calls the task switcher. The state of a task
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consists of the values of the Instruction Pointer (IP),
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consists of the values of the Instruction Pointer (IP),
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@ -97,6 +97,137 @@ follows the call of the wake-up procedure. This procedure invites
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state of task (consisting of SP ‘a RP and IP) and sets the
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state of task (consisting of SP ‘a RP and IP) and sets the
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Userpointer (UP), so that it shows this task.
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Userpointer (UP), so that it shows this task.
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#### Memory map of a task
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```
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r0 @ -> ╔════════════════════╗ ─────
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║ return stack ║ ⋀
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rp@ -> ╠═════════════════|══╣ |
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║ ⋁ ║
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║ free ║ rlen
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║ ⋀ ║
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╠═════════════════|══╣ |
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║ user area ║ ⋁
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up@ -> ╠════════════════════╣ ─────
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║ heap ║ ⋀
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heap -> ╠════════════════════╣ |
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║ (stack underflow) ║
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s0 @ -> ╠════════════════════╣
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║ stack ║
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sp@ -> ╠═════════════════|══╣ slen
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║ ⋁ ║
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║ free ║
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║ ⋀ ║
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here -> ╠═════════════════|══╣ |
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║ dictionary ║ ⋁
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╚════════════════════╝ ─────
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```
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There's a small unused area of 6 bytes between stack and heap to prevent heap
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corruption in case of a small stack underrun.
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And typically, the dictionary of any task but the main task will be empty, as
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that is where the outer interpreter is running which usally populates the
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dictionary through definitions. However, `dp` is a user variable, so each task
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has its own `here`, and if a task calls `allot`, the memory will be allocated
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in its own dictionary.
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#### Tasks' user areas
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A task is characterized by the address of its user area. The tasker's round
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robin loop consists of
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```
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task 3: jmp XXXX => task is sleeping
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│ ... │ task3 + 6
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├───────────┤
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│ jsr (wake │ task3 + 3
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├───────────┤
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┌> -> -> │ jmp XXXX │-> ┐ task3 + 0
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⋀ └───────────┘ ⋁
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| ┌ <- <- <-- <- <- <┘
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⋀ ⋁
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| | task 2: bit XXXX => task is active
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⋀ ⋁
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| | │ ... │ task2 + 6
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⋀ ⋁ ├───────────┤
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| | │ jsr (wake │ task2 + 3
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⋀ ⋁ ├───────────┤
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| └> │ bit XXXX │-> ┐ task2 + 0
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⋀ └───────────┘ ⋁
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| ┌ <- <- <-- <- <- <┘
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⋀ ⋁
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| | task 1: bit XXXX => task is active
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⋀ ⋁
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| | │ ... │ task1 + 6
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⋀ ⋁ ├───────────┤
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| | │ jsr (wake │ task1 + 3
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⋀ ⋁ ├───────────┤
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| └> │ bit XXXX │-> ┐ task1 + 0
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⋀ └───────────┘ ⋁
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└ <- <- <- <- <-- <- <- <┘
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```
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#### Overall memory map
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```
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0xFFFF -> ╔════════════════════╗
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║ I/O and ROM ║
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║ (starts at 0x8000, ║
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║ 0x9F00, 0xD000 or ║
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║ 0xFD00 depending ║
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║ on platform) ║
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limit -> ╠════════════════════╣
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║ buffers ║
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first @ -> ╠════════════════════╣
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║ (unused) ║
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r0 @ -> ╠════════════════════╣ ─────
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║ return stack ║ ⋀
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rp@ -> ╠═════════════════|══╣ |
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║ ⋁ ║
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║ free ║ rlen
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║ ⋀ ║
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╠═════════════════|══╣ |
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║ user area ║ ⋁
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up@ -> ╠════════════════════╣ ─────
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║ heap ║ ⋀
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heap -> ╠════════════════════╣ |
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║ (stack underflow) ║
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s0 @ -> ╠════════════════════╣
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║ stack ║
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sp@ -> ╠═════════════════|══╣ slen
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║ ⋁ ║
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║ free ║
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║ ⋀ ║
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here -> ╠═════════════════|══╣
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║ ║
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║ dictionary ║
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║ ║
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╠════════════════════╣ |
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║ boot area ║ ⋁
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origin -> ╠════════════════════╣ ─────
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║ 10 SYS(2064) ║
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╠════════════════════╣
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║ video memory ║
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╠════════════════════╣
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║ Kernal vars ║
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║ 6502 ZP, stack ║
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╚════════════════════╝
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```
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#### 6502 zero page
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|Label | C64 | C16 | X16 |
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|-------|-------|-------|-------|
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|N |0x0029 |0x0029 |0x0067 |
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|W |0x0021 |0x0021 |0x006F |
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|IP |0x000E |0x000E |0x005C |
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|NEXT |0x0009 |0x0009 |0x0057 |
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|SP |0x0007 |0x0007 |0x0055 |
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|Put A |0x0006 |0x0006 |0x0054 |
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|UP |0x0004 |0x0004 |0x0052 |
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|RP |0x0002 |0x0002 |0x0050 |
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## Glossary
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## Glossary
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