Fifth - virtual machine opcodes 30 – 39

1. 30: not

Stack Effect: n -- ~n
Description: Performs a bitwise NOT operation on the top value of the data stack.
Notes: Inverts all bits of the dword at the top of the stack.
Example:
```
0x0000FFFF
not
```
After execution, stack contains 0xFFFF0000.

Stack Effect: -- r
Description: Pushes the top value from the return stack onto the data stack.
Notes: Commonly used to access loop counters or subroutine parameters.
Example:
```
42
push
i
```
After execution, data stack contains 42.

Stack Effect: port -- byte
Description: Reads a byte from the specified hardware I/O port.
Notes: Uses IN instruction to read from the port. Replaces the port address with the read byte.
Example:
```
0x60  ; Keyboard port
cprt@
```

Stack Effect: byte port --
Description: Writes a byte to the specified hardware I/O port.
Notes: Uses OUT instruction to write to the port. Consumes both the byte and port address.

Example:

0x42  ; Byte to write
0x60  ; Keyboard port
cprt!

Stack Effect: -- r2
Description: Pushes the second value from the return stack onto the data stack.
Notes: Useful for accessing deeper items in the return stack without popping.
Example:
```
1 2
push push
i2
```
After execution, data stack contains 1.

Stack Effect: -- r3
Description: Pushes the third value from the return stack onto the data stack.
Notes: Allows access to the third item in the return stack for deeper subroutine contexts.
Example:
```
1 2 3
push push push
i3
```
After execution, data stack contains 1.

Stack Effect: value count -- result
Description: Shifts value left by count bit positions. Each bit position shifted left multiplies the value by 2.
Bits Shifted Out: Bits that are shifted beyond the most significant bit (bit 31) are discarded.
Count Masking: Only the low byte of count is used. On x86 processors, the actual shift amount is further masked to 5 bits (count & 0x1F), meaning the effective shift is always 0-31.

Corner Cases:

count = 0: Value unchanged.
count > 32=: Due to 5-bit masking, the effective shift is count mod 32. For example, shift by 32 results in no change, shift by 33 shifts by 1.
value = 0: Result is always 0 regardless of count.

Examples:

1  \ Value to shift
2  \ Shift amount (count)
shl

Result: 4 (0b0001 << 2 = 0b0100).

FF    \ Value: 255 (0x000000FF)
4     \ Shift by 4
shl

Result: FF0 (0x00000FF0 = 4080 decimal). The low 4 bits (F) were shifted up, and the upper bits remain zero. #+begin_example

Stack Effect: value count -- result
Description: Shifts value right by count bit positions. Each bit position shifted right divides the value by 2 (integer division).
Bits Shifted Out: Bits that are shifted beyond the least significant bit (bit 0) are discarded.
Zero Fill: This is a logical shift right (unsigned). Zeros are shifted in from the left (MSB side). This differs from an arithmetic shift right which would preserve the sign bit.
Count Masking: Only the low byte of count is used. On x86 processors, the actual shift amount is further masked to 5 bits (count & 0x1F), meaning the effective shift is always 0-31.

Corner Cases:

count = 0: Value unchanged.
count > 32=: Due to 5-bit masking, the effective shift is count mod 32. For example, shift by 32 results in no change, shift by 33 shifts by 1.
value = 0: Result is always 0 regardless of count.
Shifting by 31 always results in 0 (all bits shifted out).

Examples:

8  \ Value to shift
2  \ Shift amount (count)
shr

Result: 2 (0b1000 >> 2 = 0b0010).

1      \ Value: only bit 0 set
1      \ Shift by 1
shr

Result: 0. The single bit was shifted out and discarded.