A record is a bit pattern you define to format bytes, words, or dwords for bit-packing. The RecordName becomes a Record-TypeName that can be used create record variables.
Syntax
RecordName RECORD FieldDeclaration [, [LineBreak] FieldDeclaration ...]
Where FieldDeclaration has the following form:
FieldName:Width[ = InitialValue]
The optional LineBreak entry allows you to end a FieldDeclaration with a comma, enter an optional EndOfLine-Comment followed by a physical NewLine character, then continue the record definition on the next line. Remarks
The RecordName and FieldName entries are unique globally-scoped Identifiers that must be specified. Upon successful processing of the RECORD definition, the RecordName entry is converted to a Record-TypeName, and all FieldNames are converted to Record-FieldNames.
Each Width entry in a FieldDeclaration is specified as an Expression which must evaluate to an Absolute-ExpressionType. The cumulative value of all Width entries becomes the total RecordWidth and must not exceed 32, the size of a DWORD, the maximum size for a Record-TypeName. The Operand Size of the record becomes 1 (BYTE) if the RecordWidth is from 1 through 8, 2 (WORD) if the RecordWidth is from 9 through 16, and 4 (DWORD) if the RecordWidth is from 17 through 32. Any other value causes an error. If the total number of bits in the RecordWidth is not evenly divisible by the Operand Size, the assembler right-justifies the fields into the least-significant bit positions of the record.
When a Record-FieldName is referenced in an expression, the value returned is the shift value required to access the field. The WIDTH operator is used on the Record-FieldName to return the width of the field in bits, and the MASK operator is used to obtain the value necessary for isolating the field within the record.
The InitialValue entry contains the default value to used for the field when a record variable is allocated. If the field is at least 7 bits wide, you can initialize it to an ASCII character (for example, FIELD:7='Z').
To initialize a record, use the form:
[Identifier] Record-TypeName Expression [, Expression ...]
The Identifier entry is an optional name for the first byte, word, or dword of the reserved memory. The Record-TypeName defines the format and default field values, and is the RecordName you assigned to the record in the RECORD directive.
At least one Expression entry must be specified; additional entries are optional. The Expression must resolve to a Compound-ExpressionType, which may also be be duplicated by specifying it as a sub-expression of a Duplicated-ExpressionType. Each Compound-ExpressionType represents a single allocated record entry; each explicit sub-expression of the Compound-ExpressionType represents a field value which overrides the default InitialValue for the field given in the record definition. Example
Define the record fields; begin with the most significant fields first:
MODULE RECORD R:7, ; First field. ", LineBreak" syntax
E:4, ; may be used to split RECORD
D:5 ; definition across multiple lines
Fields are 7 bits, 4 bits, and 5 bits; the assembler gives no default value. Most significant byte first, this looks like:
RRRR RRRE EEED DDDD
To reserve its memory:
STG_FLD MODULE <7,,2> ; Initializer is a Compound-ExpressionType
This defines R=7 and D=2 and leaves E unknown; it produces 2 bytes, the least significant byte first:
02 0E
Define the record fields:
AREA RECORD FLA:8='A',FLB:8='B'
To reserve its memory:
CHAR_FLD AREA <,'P'>
This defines FLA='A' (the default) and changes FLB='P'.
To use a field in the record:
DEFFIELD RECORD X:3,Y:4,Z:9
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TABLE DEFFIELD 10 DUP(<0,2,255>)
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MOV DX,TABLE[2]
; Move data from record to register
; other than segment register
AND DX,MASK Y
; Mask out fields X and Y
; to remove unwanted fields
; The MASK of Y equals 1E00H
; 00011111000000000B (1E00H) Is the value
MOV CL,Y ; Get shift count
; 9 is the value
SHR DX,CL ; Field to low-order
; bits of register, DX is now
; equal to the value of field Y
MOV CL,WIDTH Y ; Get number of bits
; in field - 4 is the value,
; the WIDTH of Y is 4