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The STMH returns the end address of the last word stored on the heap instead of the begin address for convenience in recursive structures

* the heap starts at a fixed position in memory and grows in the same direction as the stack (away from it)
* the next free address in the heap is stored in register R4(HP)
* there are four instructions to store/load to/from the heap (STH, STMH s, LDH o, and LDMH o s where o is an offset and s a size), producing and consuming an address respectively