Terminology

C

Communications Register Unit: serial interface embedded in TMS processors

Context switch: Operation of TMS processors. The execution context is defined as the Workspace,the current execution location, and the status flags. By a context switch, the workspace pointer, the program counter, and the status register are loaded with new values corresponding to the new context. A context switch simplifies modular programming in assembly language. There are three machine lanuage operations that perform context switches: BLWP (branch and load workspace pointer), XOP (extended operation), and RTWP (return with workspace pointer). Apart from those, two more kinds of context switches appear: the initial context switch to the values stored at >0000, and interrupts that are services by an ISR.

CRU: short for Communications Register Unit

D

Device Service Routine: Program that is provided for accessing a peripheral device; would be called a driver in common terminology. The DSR defines device names and functions to operate the device on a higher level, abstracting from the hardware details. There are utility functions in the BASIC ROM and for assembly language programs to facilitate accessing the DSR of a device given by name (DSRLNK). The DSR is usually stored in a ROM circuit that is included on the PCB of the expansion card. Each card may be selected by a CRU operation which leads to the DSR being mapped into the CPU address space. The common region for DSRs is >4000 to >5fff. On the Geneve, DSRs are also included as parts of the MDOS operating system, replacing the fixed DSRs on the ROMs.

DSR: see Device Service Routine

E

Endianness: Determines the order of bytes within words. Big-endian architectures have the higher-order byte at address x and the lower-order byte at x+1. Little-endian system have the lower-order byte at x and the higher-order byte at x+1. May also determine the bit order in a byte or word, where a big-endian bit order means that bit 0 has highest value, while bit 7 or bit 15 has lowest value. The TI systems are big-endian with respect to both multi-byte words and bit order.

F

Firehose cable: Nickname for the Flex Cable Interface

Flex Cable Interface: Card, cable, and plug that connect the TI-99/4(A) console with the Peripheral Expansion Box. The Flex Cable Interface's card was usually plugged into slot 1 of the P-Box.

I

ISR: see Interrupt Service Routine

P

P-Box: short for Peripheral Expansion System Box or Peripheral Expansion Box

W

Workspace: Set of 16 workspace registers. The TMS9900/9995 processors do not have user-defined registers within the processor but make use of external RAM for their registers. This allows to use context switches, but also slows down execution considerably. The current workspace is defined by the processor register WP (workspace pointer). Choosing a new workspace just amounts to loading a new value into this register. The workspace pointer points to address of the high byte of register 0.