RST is simply the opcode chosen to represent the Restart instruction.
The lowest priority interrupt in the 8085 microprocessor is INTR, unless you also consider the software interrupts, RST 0 through RST 7, which are even lower.
The 5.5 in RST 5.5 means that the interrupt vector is located between RST 5 and RST 6.
RSTn(0-7) is software interrupt & RST(7.5,6.5,5.5) are INTEL's 8085 maskable interrupts their priority is respectively 7.5,6.5,5.5.RSTn.5 is hardware intterrupt and it is controlled by the external pin available on the chip of 8085. it is used to interrupt the microprocessor from its normal execution and tends to execute some ISR(interrupt service routine)
RST 5.5 is level triggered interrupt & maskable also. it can be masked by using SIM intrruction. Vector address of the RST 5.5 interrupt is 002Ch
RST 5.5 is an interrupt request pin (pin 9) on the 8085. It is sampled, along with other interrupt requests, one clock cycle before ALE on the falling edge of CLK. When enabled, and having priority, it causes an interrupt vector sequence using the RST 5.5 instruction, going to location 002CH.
The 8085 has five interrupts, INTR, RST5.5, RST6.5, RST7.5, and TRAP. It also has eight software interrupts, RST0, RST1, ..., RST7. The INTR interrupt requires a hardware response that is an opcode. Usually, the opcode is either a CALL instruction, in which case the interrupt vector can go anywhere in memory, or it is an RST instruction, in which case the vector is based on a table in low memory. In the case of RST instructions, either directly or via INTR, or the RSTx.5 interrupts, you simply multiply the interrupt number by 8 to get the vector address. The following table presents the vector addresses for all possible interrupts... RST 0 - 00H RST 1 - 08H RST 2 - 10H RST 3 - 18H RST 4 - 20H TRAP - 24H RST 5 - 28H RST5.5 - 2CH RST 6 - 30H RST 6.5 - 34H RST 7 - 38H RST 7.5 - 3CH
The name, RST6.5, was chosen because the interrupt vector lies halfway between RST 6 and RST 7, specifically at address 0034H.
The INTA- (Interrupt Acknowledge) pin on the 8085 is an acknowledge of the INTR (Interrupt Request). It has the same timing as RD-, and external hardware is expected to provide an opcode, usually a CALL or an RST instruction, in response to INTA-.
in RST interrupt , RST STAND FOR
RST OF TRAP is 24h(hexadecimail) address.
The interrupt vector table in the 8085 is a region of low memory that contains the target addresses for the RST instructions. RST can be invoked by the program, by an INTR request which provides an RST x instruction in response to INTA, or by one of the four direct interrupt pins, TRAP, RST5.5, RST6.5, and RST7.5 Each of these interrupt sequences place the PC on the stack, and then execution goes to one of the vectors. The vectors are as follows... RST 0: 0000H RST 1: 0008H RST 2: 0010H RST 3: 0018H RST 4: 0020H TRAP: 0024H RST 5: 0028H RST5.5: 002CH RST 6: 0030H RST6.5: 0034H RST7: 0038H RST7.5: 003CH
A hardware interrupt is initiated from a hardware event, such as a byte received from a serial port. It is asynchronous to the running program, i.e. it can occur at any time. An internal, or software, interrupt is initiated by the running program, thus it is under the control of the running program. In the 8085, for instance, the RST instructions are software interrupts. Note, however, that external interrupts can initiate a software RST instruction, so the two events can appear to be the same kind of event. The biggest difference is the handling of the interrupt enable (IE) flag. In the hardware interrupt, the IE flag is turned off when processing the interrupt, and it is turned on right before the interrupt service routine executes the final return instruction. The software interrupt, on the other hand, has nothing to do with IE. As a result, the code must be able to tell the difference.