Texas Instruments MSP50C6xx manual 8 Tag/Flag Bits

Instruction Syntax and Addressing Modes

4.3.8Tag/Flag Bits

The words TAG and flag may be used interchangeably in this manual. The TAG bit is the 17th bit of a word of data memory. There are 640 words of RAM, each 17 bits wide, on the C614. Therefore, there are 640 TAG bits on the C614. When an instruction of the format,

MOV accumulator, RAM

is performed, the STAT register is affected by various properties of this trans- fer. The TAG bit of the RAM location is copied into the TAG bit of the STAT reg- ister during such transfers.

The TAG bit can be modified using several instructions: STAG, RTAG, SFLAG, RFLAG. There are subtle differences between these instructions that the user must understand before using them. The first difference between the xTAG and xFLAG instructions is the addressing.

STAG *0x0000 ;sets the TAG bit of RAM word zero

RTAG *0x0002 ;clears the TAG bit of RAM word one

STAG *0x0002 * 2 ;sets the TAG bit of RAM word two

STAG and RTAG use RAM byte addresses to specify which TAG to set or clear. This immediately causes confusion since there are 1280 bytes and only 640 TAGs. What happens when an odd byte is used to set a tag with STAG?

STAG *0x0001 ;sets the TAG bit of RAM word zero

STAG *0x0003 ;sets the TAG bit of RAM word one

STAG *0x0005 * 2 ;sets the TAG bit of RAM word five

All word boundaries in RAM start at even numbers, RAMeven. If an odd byte, RAMeven + 1 is used to set a TAG, then the TAG for RAMeven is set. Thus,

STAG *0x0000

STAG *0x0001

are functionally equivalent.

As a sharp contrast, the SFLAG and RFLAG instructions use RAM word ad- dresses to specify which TAG to set or clear.

SFLAG *0x0000 ;sets the TAG bit of RAM word zero

SFLAG *0x0001 ;sets the TAG bit of RAM word one

Another difference between the xTAG and xFLAG instructions is the addres- sing modes. STAG and RTAG can use {adrs} addressing modes. This in- cludes, direct, short relative, relative to R5, long relative, and indirect addres- sing modes. This affects the number of clock cycles it takes to execute xTAG instructions.

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