ST10F280-AB

2/4

2.2 - MAC.9 - CoCMP Instruction Inverted Operands

The ST10 Family Programming Manual describes the CoCMP instruction as: subtracts a 40-bit signed

operand from th 40-bit accumulator content (acc - op2\op1), and updates the N, Z and C flags in the MSW

register, leaving the accumulator unchanged. On the device the reverse operation (op2\op1 - acc) has

been implemented in the Mac Unit. Therefore, the N and C flags are set according to the reverse

operation (Z flag is not affected).

Workaround:

Change interpretation of the N and C flags in the MSW register.

Example:

MOV

R12, #07h

MOV

R13, #06h

MOV

R14, #0

CoLOAD

R14, R12

; Accumulator = 70000h

CoCMP

R14, R13

; Compares 70000h to 60000h

Here the content of MSW is 0500h, i.e. C = 1, Z = 0 and N = 1.

To test if the Accumulator was greater than or equal the compared value, the "normal" test, according to

the description in the ST10 Programming Manual, would be:

JNB

MSW.10, Greater

; If C flag cleared, then greater than or equal

With the implementation, this test does not provide the expected result.

To obtain the correct comparison, use instead:

JB

MSW.10, Greater

; C flag set: 60000h < 70000h (60000h-70000h implemented)

; i.e. the accumulator is greater than or equal compared value

2.3 - MAC.10 - E Flag Evaluation for CoSHR and CoASHR Instructions when Saturation Mode is Enabled

The Logical and the Arithmetic Right Shift instructions (CoSHR/CoASHR) are specififed not to be affected

by the saturation mode (MS bit of the MCW register): the shift operation is always made on the 40 bits of

the accumulator. The result shifted in the accumulator is never saturated. Only when the saturation mode

is enabled, the evaluation of the E Flag (in the MSW register) is erroneous.

Comment to the example:

In the example below (Table 1), the E Flag is kept cleared however MAE is used: bit 0 of MAE has been

shifted into bit 15 of MAH. The MAE part has been used and it contents significant bits but the E Flag has

not been set.

The content of the flags is given after the execution of the instruction.

Workaround:

If the MAE flag is used, the saturation mode must be disabled before running Logical and/or Arihmetic

Right Shift instructions and re-enable just after.

Table 1 : MAC.10 Example

MS Bit is Set, Saturation Mode is Enabled

Status of Flags After Instruction Execution

Code

Accumulator Value

(Hexa.)

SL

E

SV

C

Z

N

Remark

MOV

R5, #5555h

--

----

----

-

-----

-

CoLOAD

R5, R5

00

5555

5555

0

00000

Right

NOP

00

5555

5555

0

00000

Right

MOV

MSW, #007Fh

7F

5555

5555

0

00000

Right

NOP

7F

5555

5555

0

00000

Right

CoSHR

#1

3F

AAAA

AAAA

0

0*

0000

*E is wrong