Texas Instruments MSP50C6xx manual Initial Processor State for String Instructions

String Instructions

4.8 String Instructions

Class 1, 2, 3, and 6 instructions can have string modes. During the execution of string instruction, STR register value plus 2 is assumed as string length. An accumulator string is a group of consecutive accumulators spanning from An to the next N consecutive accumulators (N is the length of the string). The STR register should be loaded with N–2 to define a string length, N. A value of zero in the STR register defines a string length of 2 (string length 1 means the instruction is not in string mode). Arithmetic string instructions treat the string as an N word arithmetic value. The result is also an arithmetic value of the same length. Conditionals are set as they would be set without string mode. Comparing two strings is equivalent to comparing each bit of the string. The accumulator status is modified representing the outcome of the entire operation. Examine the following examples.

Table 4–43. Initial Processor State for String Instructions

Registers (register# = value)

AP0 = 2	AP1 = 21 (0x15)		AP2 = 11 (0x0B)	AP3 = 29 (0x1D)
AC0 =	AC1 =		AC2 =	AC3 =
AC4 =	AC5 =		AC6 =	AC7 =
AC8 =	AC9 =		AC10 =	AC11 = 0xAAAA
AC12 = 0xAAAA	AC13 = 0xAAAA		AC14 = 0xAAAA	AC15 = 0xAAAA
AC16 =	AC17 =		AC18 =	AC19 =
AC20 =	AC21 = 0x1223		AC22 = 0xFBCA	AC23 = 0x233E
data memory (*address = data)
*0x0200 = 0x12AC	*0x0201	= 0xEE34	*0x0202 = 0x9086	*0x0203	= 0xCDE5
program memory (*address = data)
*0x1400 = 0x0123	*0x1401	= 0x4567	*0x1402 = 0x89AB	*0x1403	= 0xCDEF
*0x1404 = 0xFEDC	*0x1405	= 0xBA98	*0x1404 = 0x7654	*0x1405	= 0x3210
	Example 4.8.1		MOV STR, 4–2;	string length = 2

MOVS A0, 0x1400

Refer to initial the processor state in Table 4–43. A0 points to AC2. Consider a program memory location string of length 4 at 0x1400 = 0xCDEF89AB45670123. STR equal to 4–2=2, defines a string length of 4. Final result, AC2=0x0123, AC3=0x4567, AC4=0x89AB, and AC5=0xCDEF,

Example 4.8.2	MOV STR, 3–2;	string	length = 3
	ADDS A1~, A1,	*0x0200

Refer to the initial processor state in Table 4–43. A1 is AC21, A1~ is AC5, the

Assembly Language Instructions 4-55