
Enhanced Multiply-Accumulate Unit (EMAC)
Freescale Semiconductor
4-11
•
If R0.L is 0x8000, R0 is half-way between two 16-bit numbers. In this case, rounding is based on
the lsb of R0.U, so the result is always even (lsb = 0).
— If the lsb of R0.U equals 1 and R0.L equals 0x8000, the number is rounded up.
— If the lsb of R0.U equals 0 and R0.L equals 0x8000, the number is rounded down.
This method minimizes rounding bias and creates as statistically correct an answer as possible.
The rounding algorithm is summarized in the following pseudocode:
if R0.L < 0x8000
then Result = R0.U
else if R0.L > 0x8000
then Result = R0.U + 1
else if lsb of R0.U = 0
/* R0.L = 0x8000 */
then Result = R0.U
else Result = R0.U + 1
The round-to-nearest-even technique is also known as convergent rounding.
4.3.1.2
Saving and Restoring the EMAC Programming Model
The presence of rounding logic in the EMAC output datapath requires that special care during the EMAC’s
save/restore process. In particular, any result rounding modes must be disabled during the save/restore
process so the exact bit-wise contents of the EMAC registers are accessed. Consider the memory structure
containing the EMAC programming model:
struct macState
{
int acc0;
int acc1;
int acc2;
int acc3;
int accext01;
int accext02;
int mask;
int macsr;
} macState;
The following assembly language routine shows the proper sequence for a correct EMAC state save. This
code assumes all Dn and An registers are available for use, and the memory location of the state save is
defined by A7.
EMAC_state_save:
move.l macsr,d7
; save the macsr
clr.l d0
; zero the register to ...
move.l d0,macsr
; disable rounding in the macsr
move.l acc0,d0
; save the accumulators
move.l acc1,d1
move.l acc2,d2
move.l acc3,d3
move.l accext01,d4
; save the accumulator extensions
move.l accext23,d5
move.l mask,d6
; save the address mask
movem.l #0x00ff,(a7)
; move the state to memory
This code performs the EMAC state restore:
EMAC_state_restore:
MCF5329 Reference Manual, Rev 3
Summary of Contents for MCF5329
Page 106: ...ColdFire Core 3 32 Freescale Semiconductor MCF5329 Reference Manual Rev 3...
Page 148: ...Cache 5 22 Freescale Semiconductor MCF5329 Reference Manual Rev 3...
Page 154: ...Static RAM SRAM 6 6 Freescale Semiconductor MCF5329 Reference Manual Rev 3...
Page 184: ...Power Management 8 18 Freescale Semiconductor MCF5329 Reference Manual Rev 3...
Page 204: ...Reset Controller Module 10 8 Freescale Semiconductor MCF5329 Reference Manual Rev 3...
Page 220: ...System Control Module SCM 11 16 Freescale Semiconductor MCF5329 Reference Manual Rev 3...
Page 228: ...Crossbar Switch XBS 12 8 Freescale Semiconductor MCF5329 Reference Manual Rev 3...
Page 268: ...General Purpose I O Module 13 40 Freescale Semiconductor MCF5329 Reference Manual Rev 3...
Page 392: ...SDRAM Controller SDRAMC 18 30 Freescale Semiconductor MCF5329 Reference Manual Rev 3...
Page 436: ...Fast Ethernet Controller FEC 19 44 Freescale Semiconductor MCF5329 Reference Manual Rev 3...
Page 594: ...FlexCAN 23 30 Freescale Semiconductor MCF5329 Reference Manual Rev 3...
Page 678: ...Pulse Width Modulation PWM Module 26 22 Freescale Semiconductor MCF5329 Reference Manual Rev 3...
Page 684: ...Watchdog Timer Module 27 6 Freescale Semiconductor MCF5329 Reference Manual Rev 3...
Page 704: ...DMA Timers DTIM0 DTIM3 29 12 Freescale Semiconductor MCF5329 Reference Manual Rev 3...
Page 754: ...UART Modules 31 34 Freescale Semiconductor MCF5329 Reference Manual Rev 3...
Page 770: ...I2 C Interface 32 16 Freescale Semiconductor MCF5329 Reference Manual Rev 3...
Page 866: ...Debug Module 36 50 Freescale Semiconductor MCF5329 Reference Manual Rev 3...