45205 Parallax Inc, 45205 Datasheet - Page 166
45205
Manufacturer Part Number
45205
Description
SX TECH BOARD
Manufacturer
Parallax Inc
Type
MCUr
Datasheet
1.45180.pdf
(184 pages)
Specifications of 45205
Contents
Board
For Use With/related Products
SX MCUs
For Use With
28825 - MINI SOUND PLAYER SOUNDPAL130-28029 - KIT PARTS SMART SENSORS
Lead Free Status / Rohs Status
Lead free / RoHS Compliant
- Current page: 166 of 184
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15 Appendix E: SX Data Sheet
15.2.20 The Pop
When a return instruction is executed, the stack is popped. Specifically, the content of Stack 1 is copied
to the program counter and the content of each address in the stack is copied to the next higher level.
Stack 1 gets the value that was in Stack 2, etc. until Stack 7 is overwritten with the contents of Stack 8.
Consequently, the contents that were in Stack 8 are now duplicated in Stack 7.
15.2.21 Stack Overflow
As mentioned before, the stack can store up to eight return addresses (with Stack Extend on), or up to
two return addresses by default. With each push, the stack stores another address. When the stack is
full, the next push results in an overflow. The first time the stack is pushed into an overflow condition,
the first address pushed is lost forever. If the stack were to be pushed again, the second address pushed
would be lost also. A stack overflow condition inevitably leads to unintentional infinite loops or bizarre
looping actions in your program. Care should be taken to ensure a stack overflow does not ever occur.
15.2.22 Stack Underflow
When the stack is popped more times than it has been pushed, a stack underflow occurs. Since a stack
underflow causes unknown addresses to be stored in PC, a program may perform bizarre looping
actions, such as a jump to unused program memory.
15.2.23 Returns
There are five different return instructions available on the SX. The RET (return) instruction simply
pops the stack thereby setting the program counter to the instruction that followed the call. The RETW
(return with literal in w) instruction behaves the same way but loads W with the literal value specified.
RETP pops the stack and updates the page select bits to point to the page returned to. RETI pops the
Page 166 SX-Key/Blitz Development System Manual 2.0 Parallax, Inc.
Figure 35 - The Pop
PC<10:0>
Stack 1
Stack 2
Stack 3
Stack 4
Stack 5
Stack 6
Stack 7
Stack 8
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