Lecture 3 - Instruction Set - Al Exception Handling (2) 9/20/6 Lecture 3 - Instruction Set - Al

Lecture 3 - Instruction Set - Al 68000 Exception Handling The 68000’s exception vector table 9/20/6 Lecture 3 - Instruction Set - Al

Exception vector table 256 longwords in low memory $00 0000 to $00 03FF (1024 bytes) Allow specialized routine to handle each exception type Some 8-bit uprocessors even have a small table 9/20/6 Lecture 3 - Instruction Set - Al

Exception Vector Table System reserves low Memory for the table If some vectors not used then no handler Rather than no pointer, point to a Spurious exception handler 9/20/6 Lecture 3 - Instruction Set - Al

Lecture 3 - Instruction Set - Al How to implement it? First answer is MUST be in ROM – 1st 2 longwords are initial supervisor stack pointer and initial program counter $00 0000 to ? These must be there on start up ROM is good as they will be there at startup ROM is also bad as not easily modified Changes require a new ROM 9/20/6 Lecture 3 - Instruction Set - Al

Lecture 3 - Instruction Set - Al Solutions Fixed vector in ROM points to a second vector in RAM that points to routine WHY? Not possible to get a 16 byte ROM Possibly overlay a ROM and RAM at same addresses? When address is in range $00 0000 to $00 0007 ROM is being addressed Otherwise RAM 9/20/6 Lecture 3 - Instruction Set - Al

Lecture 3 - Instruction Set - Al Overlay scheme When reading $00 0000 or $00 0004 actually read $00 1000 or $00 1004 9/20/6 Lecture 3 - Instruction Set - Al

Implementation of Overlay When location $00 0000 or $00 0004 is address ROM is selected 9/20/6 Lecture 3 - Instruction Set - Al

Lecture 3 - Instruction Set - Al 2nd Method – Shadow ROM RAM and ROM are located in the same address space At startup POR* selects ROM POR* does a reset of FF1 On Reads ROM is selected, on Writes RAM Once vector table copied FF1 is set so future reads are from RAM 9/20/6 Lecture 3 - Instruction Set - Al

Lecture 3 - Instruction Set - Al Implementation ROM is addressed until RAM* becomes low After RAM* goes low the RS FF enables the RAM RAM* would be a signal from an I/O port 9/20/6 Lecture 3 - Instruction Set - Al

Processing an Vectored Interrupt Complete instruction currently executing Stack PC Continue according to protocol 9/20/6 Lecture 3 - Instruction Set - Al

Lecture 3 - Instruction Set - Al IACK cycle During IACK device provides vector number The exception handler starts execution 9/20/6 Lecture 3 - Instruction Set - Al

Hardware interface of device 68000 family peripheral having IACK input Note that IACK has unique Function Code, FC7, i.e., 111 9/20/6 Lecture 3 - Instruction Set - Al

Autovectored interrupt Older devices and non-family devices unable to respond with appropriate vector number Another pin, VPA*, Valid Peripheral Address is used to signal this. Vector numbers 25-31 are reserved for autovectored interrupts on IRQ1* to IRQ7* Avoids having a lot of glue logic for these older devices 9/20/6 Lecture 3 - Instruction Set - Al

Timing of autovectored interrupts Example IRQ2* asserted Followed by VPA* being asserted Vector 26 used to respond Autovectored are 25 to 31 in vector table 9/20/6 Lecture 3 - Instruction Set - Al

Autovectored Hardware System must have vectored interrupt hw Autovectored needed only if 6800 family peripherals used 9/20/6 Lecture 3 - Instruction Set - Al