ST9 datasheet(35/146 Pages) STMICROELECTRONICS

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ST9 USER GUIDE

If the mask is a byte whose binary value is 11000000, only the left-most two bits of the un-

known value will be tested, and a later branch if zero will be taken or not according only to

these bits. As shown below, the same mask is used for two values that differ only by one bit:

Two more instructions, tcm and tcmw, work essentially the same way, but they take the com-

plement of the value to be tested before ANDing it with the mask, as follows:

tcm value, mask

The same two cases will provide the following results:

The jump would be taken if the byte to be tested had two 1’s in its most significant two bits, for

example 11110101.

3.3.1.3 Push and Pop

Since there are two stacks, there are two kinds of push and pop instructions. The mnemonics

push, pushw, pop and popw act on the system stack, which can be either in the register space

or in the memory space. The mnemonics pushu, pushuw, popu and popuw act on the user

stack, that can also be either in the register space or the memory space. The stack pointer

used in each case is the SSPR or the USPR register pair respectively. The stack pointers are

always decremented before writing on pushing, and they are incremented after reading on

popping. Thus the stack pointer always points to the last byte written. This is worth knowing if

you need to manipulate the stack contents.

The operands to be pushed can be a register, a pair of registers or an immediate value:

000110 101

Byte to be tested

0110

101

110000 000

Mask used for testing

1000

000

000000 000

Result of the logical AND operation

0000

000

jump taken

Result of the “jump if zero”

jump not taken

000110 101

Byte to be tested

0110

101

111001 010

Complement of the byte to be tested

1001

010

110000 000

Mask used for testing

1000

000

110000 000

Result of the logical AND operation

1000

000

jump not taken

Result of the “jump if zero”

jump not taken

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