SH7615 Datasheet, PDF (80/925 Pages) Renesas Technology Corp – 32-Bit RISC Microcomputer SuperH
RISC engine Family/SH7600 Series

2.1.5 Notes on Guard Bits and Overflow Treatment
DSP unit data operations are fundamentally performed in 32 bits, but these operations are always
executed with a 40-bit length including the 8-bit guard section. When the guard bit section does
not match the value of the 32-bit section MSB, the operation result is treated as an overflow. In
this case, the N bit indicates the correct status of the operation result regardless of the existence or
not of an overflow. This is so even if the destination operand is a 32-bit length register. The 8-bit
section guard bits are always presupposed and each status flag is updated.
When place overflows occur so that the correct result cannot be displayed even when the guard
bits are used, the N flag cannot indicate the correct status.
2.1.6 Initial Values of Registers
Table 2.3 lists the values of the registers after reset.
Table 2.3 Initial Values of Registers
Classification Register
Initial Value
General registers R0 to R14
Undefined
R15 (SP)
Value of the SP in the vector address table
Control registers SR
Bits I3 to I0 are 1111 (H'F), the reserved bits, RC,
DMY, and DMX are 0, and other bits are undefined
RS
Undefined
RE
GBR
Undefined
VBR
H'00000000
MOD
Undefined
System registers MACH, MACL, PR
Undefined
PC
Value of the PC in the vector address table
DSP registers
A0, A0G, A1, A1G, M0, Undefined
M1, X0, X1, Y0, Y1
DSR
H'00000000
Rev. 2.00, 03/05, page 42 of 884