Modular Subtract Register Usage - Motorola DigitalDNA MPC180E User Manual

ECC Routines
7.3.5 ECC F Modular Subtract
p
Modular subtraction may be performed on any two vectors loaded into A (A0–A3) and B
(B0–B3), where both of these vectors are less than the value stored in the modulus register
N (N0–N3). This is accomplished by computing A-B if A > B or A-B+N if A < B. The
results are stored in the respective B register. For ECC functionality, this function is used
by the point add and point double routines but is available to the host interface. This
function operates with a minimum of four digits (Modsize = 3).
Before this function is initialized, the A, B and N register pointers must be set in the control
register which indicate which sub-registers (A0, B0, A1, B1, etc.) are the targeted operands.
See Table 7-2 for a detailed description. Once this is performed, the host processor may
successfully initiate this function.
Computation C = D - E mod N, where D, E, and C are integers and are less than N
Entry name modularsubtract;
Entry address 009h(modularsubtract)
Pre-conditions A0-3 = D (integer, exact A-location pre-selected in Control Register)
B0-3 = E (integer, exact B-location pre-selected in Control Register)
N0-3 = prime p (modulus) of the ECC system
Post-conditions B0-3 = results of modular subtraction stored where the B operand was located
Unless explicitly noted, all other registers are not guaranteed to be any particular value.
Special The function operates the same regardless of whether or not the operands are in the
conditions Montgomery residue system.
Initial Condition
modulus N (if N0 selected)
Figure 7-8. Modular Subtract Register Usage
7-14
PRELIMINARY—SUBJECT TO CHANGE WITHOUT NOTICE
Table 7-9. Modular Subtract
Modular Subtract
B3
B2
B1
B0
E (if B0 selected)
A3
A2
A1
D (if A0 selected)
A0
N3
N2
N1
N0
'1' - ECC enabled ECC
EXP(k)
XYZ
F2M
'0' - F enabled
p
regAsel
set (00, 01, 10, 11)
regBsel
set (00, 01, 10, 11)
regNsel
set (00, 01, 10, 11)
Modsize
set
EXP(k)_SIZE
MPC180E Security Processor User's Manual
Final Condition
?
?
?
C (if B0 selected)
?
?
?
modulus N (if N0 selected)
same
same
same
same
same
same