Bootstrap init
Pointers
0 | 256 | SP |
1 | -1 | LCL |
2 | -2 | ARG |
3 | -3 | THIS |
4 | -4 | THAT |
Stack
256 | ??? |  | ←SP
This is how my bootstrap code initializes the pointers before calling Sys.init(). Setting the LCL, ARG, THIS and THAT pointers to known illegal values helps identify when a pointer is used before it is initialized. (If you are running the NestedCall test without bootstrap code, you will not see this state.)

Entry to Sys.init()
Pointers
0 | 261 | SP |
1 | 261 | LCL |
2 | 256 | ARG |
3 | -3 | THIS |
4 | -4 | THAT |
Stack
256 | * | Return IP | ←ARG
257 | -1 | Saved LCL |
258 | -2 | Saved ARG | Sys.init
259 | -3 | Saved THIS | frame
260 | -4 | Saved THAT |
261 | ??? |  | ←LCL, SP
This is how NestedCall.tst initializes the pointers and stack. This is what RAM looks like after my bootstrap calls Sys.init(). (If your VM translation includes the bootstrap, the -1 through -4 values may be different if your bootstrap initializes them.)

Entry to Sys.main()
Pointers
0 | 266 | SP |
1 | 266 | LCL |
2 | 261 | ARG |
3 | 4000 | THIS |
4 | 5000 | THAT |
Stack
256 | * | Return IP |
257 | -1 | Saved LCL |
258 | -2 | Saved ARG | Sys.init
259 | -3 | Saved THIS | frame
260 | -4 | Saved THAT |
261 | * | Return IP | ←ARG
262 | 261 | Saved LCL |
263 | 256 | Saved ARG | Sys.main
264 | 4000 | Saved THIS | frame
265 | 5000 | Saved THAT |
266 | ??? |  | ←LCL, SP

After Sys.main() prologue
Pointers
0 | 271 | SP |
1 | 266 | LCL |
2 | 261 | ARG |
3 | 4000 | THIS |
4 | 5000 | THAT |
Stack
256 | * | Return IP |
257 | -1 | Saved LCL |
258 | -2 | Saved ARG | Sys.init
259 | -3 | Saved THIS | frame
260 | -4 | Saved THAT |
261 | * | Return IP | ←ARG
262 | 261 | Saved LCL |
263 | 256 | Saved ARG | Sys.main
264 | 4000 | Saved THIS | frame
265 | 5000 | Saved THAT |
266 | 0 | local 0 | ←LCL
267 | 0 | local 1 |
268 | 0 | local 2 |
269 | 0 | local 3 |
270 | 0 | local 4 |
271 | ??? |  | ←SP
The function prologue is the assembly language code generated for the "function" VM command.

Entry to Sys.add12(123)
Pointers
0 | 277 | SP |
1 | 277 | LCL |
2 | 271 | ARG |
3 | 4001 | THIS |
4 | 5001 | THAT |
Stack
256 | * | Return IP |
257 | -1 | Saved LCL |
258 | -2 | Saved ARG | Sys.init
259 | -3 | Saved THIS | frame
260 | -4 | Saved THAT |
261 | * | Return IP |
262 | 261 | Saved LCL |
263 | 256 | Saved ARG | Sys.main
264 | 4000 | Saved THIS | frame
265 | 5000 | Saved THAT |
266 | 0 | local 0 |
267 | 200 | local 1 |
268 | 40 | local 2 |
269 | 6 | local 3 |
270 | 0 | local 4 |
271 | 123 | argument 0 | ←ARG
272 | * | Return IP |
273 | 266 | Saved LCL | Sys.add12
274 | 261 | Saved ARG | frame
275 | 4001 | Saved THIS |
276 | 5001 | Saved THAT |
277 | ??? |  | ←LCL, SP

Before Sys.add12() return
Pointers
0 | 278 | SP |
1 | 277 | LCL |
2 | 271 | ARG |
3 | 4002 | THIS |
4 | 5002 | THAT |
Stack
256 | * | Return IP |
257 | -1 | Saved LCL |
258 | -2 | Saved ARG | Sys.init
259 | -3 | Saved THIS | frame
260 | -4 | Saved THAT |
261 | * | Return IP |
262 | 261 | Saved LCL |
263 | 256 | Saved ARG | Sys.main
264 | 4000 | Saved THIS | frame
265 | 5000 | Saved THAT |
266 | 0 | local 0 |
267 | 200 | local 1 |
268 | 40 | local 2 |
269 | 6 | local 3 |
270 | 0 | local 4 |
271 | 123 | argument 0 | ←ARG
272 | * | Return IP |
273 | 266 | Saved LCL | Sys.add12
274 | 261 | Saved ARG | frame
275 | 4001 | Saved THIS |
276 | 5001 | Saved THAT |
277 | 135 | Return value | ←LCL
278 | ??? |  | ←SP

After Sys.add12() return
Pointers
0 | 272 | SP |
1 | 266 | LCL |
2 | 261 | ARG |
3 | 4001 | THIS |
4 | 5001 | THAT |
Stack
256 | * | Return IP |
257 | -1 | Saved LCL |
258 | -2 | Saved ARG | Sys.init
259 | -3 | Saved THIS | frame
260 | -4 | Saved THAT |
261 | * | Return IP | ←ARG
262 | 261 | Saved LCL |
263 | 256 | Saved ARG | Sys.main
264 | 4000 | Saved THIS | frame
265 | 5000 | Saved THAT |
266 | 0 | local 0 | ←LCL
267 | 200 | local 1 |
268 | 40 | local 2 |
269 | 6 | local 3 |
270 | 0 | local 4 |
271 | 135 | Return value |
272 | ??? |  | ←SP

Before Sys.main() return
Pointers
0 | 272 | SP |
1 | 266 | LCL |
2 | 261 | ARG |
3 | 4001 | THIS |
4 | 5001 | THAT |
Stack
256 | * | Return IP |
257 | -1 | Saved LCL |
258 | -2 | Saved ARG | Sys.init
259 | -3 | Saved THIS | frame
260 | -4 | Saved THAT |
261 | * | Return IP | ←ARG
262 | 261 | Saved LCL |
263 | 256 | Saved ARG | Sys.main
264 | 4000 | Saved THIS | frame
265 | 5000 | Saved THAT |
266 | 0 | local 0 | ←LCL
267 | 200 | local 1 |
268 | 40 | local 2 |
269 | 6 | local 3 |
270 | 0 | local 4 |
271 | 246 | Return value |
272 | ??? |  | ←SP

After Sys.main() return
Pointers
0 | 262 | SP |
1 | 261 | LCL |
2 | 256 | ARG |
3 | 4000 | THIS |
4 | 5000 | THAT |
Stack
256 | * | Return IP | ←ARG
257 | -1 | Saved LCL |
258 | -2 | Saved ARG | Sys.init
259 | -3 | Saved THIS | frame
260 | -4 | Saved THAT |
261 | 246 | Return value | ←LCL
262 | ??? |  | ←SP

In Sys.init() halt loop
Pointers
0 | 261 | SP |
1 | 261 | LCL |
2 | 256 | ARG |
3 | 4000 | THIS |
4 | 5000 | THAT |
Stack
256 | * | Return IP | ←ARG
257 | -1 | Saved LCL |
258 | -2 | Saved ARG | Sys.init
259 | -3 | Saved THIS | frame
260 | -4 | Saved THAT |
261 | ??? |  | ←LCL, SP