4.4 Tcode Instruction Format

Tcode instructions come in two flavours. The first sort has no inline arguments and is one byte long. The second sort has inline arguments. Its length is described in the flags field of the opcode. The first flavour has the following format:

The AR field of flavour-1 instructions must be zero, thereby indicating that the instruction has no inline arguments. The entire instruction is contained in a single byte. The C bits 0 through 6 contain the opcode of the instruction. Flavour-2 instructions have a one-byte flags prefix attached to the opcode:

The flags field of flavour-2 instructions contains the following additional fields:

An AR flag value of 1 indicates that this byte holds the flags of a flavour-2 instruction. NA holds the number of arguments of this instruction. Tcode instructions may have zero, one, or two arguments. However, zero-argument instructions are typically of flavour-1. If the SF flag is set, the last argument of this instruction is a string. L1 and L2 specify the length of operand 1 and operand 2 repectively. Each length field holds the number of bytes used to represent the operand minus 1, so an Lx value of zero denotes a single-byte operand. Operands must be sign-extended to machine word size as described earlier in this chapter.

The characters of string operands are stored in the bytes following the last argument of an instruction. For example, the IPROC statement

IPROC 128 7 "t3x_bpw"

has 2 arguments, the second one being a string. It would be encoded as follows: