p19: Add a proposal for l.nop semihosting

[stffrdhrn]

Formally document the l.nop arguments in the spec.

#define NOP_NOP          0x0000      /* Normal nop instruction */
#define NOP_EXIT         0x0001      /* End of simulation */
#define NOP_REPORT       0x0002      /* Report the value of r3 */
#define NOP_PUTC         0x0004      /* Sim putc instruction */
#define NOP_CNT_RESET    0x0005      /* Reset statistics counters */
#define NOP_GET_TICKS    0x0006      /* Get # ticks running, 64-bit result */
#define NOP_GET_PS       0x0007      /* Get picosecs/cycle, 32-bit result */
#define NOP_TRACE_ON     0x0008      /* Turn on tracing */
#define NOP_TRACE_OFF    0x0009      /* Turn off tracing */
#define NOP_RANDOM       0x000a      /* Return 4 random bytes, 32-bit result */
#define NOP_OR1KSIM      0x000b      /* Return non-zero if this is Or1ksim */
#define NOP_EXIT_SILENT  0x000c      /* End of simulation, quiet version */
#define NOP_RESET        0x000d      /* Causes the simulator to reset */

This is needed for up streaming support for things like exit and reset into qemu.

Example emails:

• 2015 [PATCH] openrisc: terminate qemu process upon receiving a halt signal.
• 2017 [PATCH 2/7] target/openrisc: add shutdown logic
• 2022 [PATCH 2/7] target/openrisc: add shutdown logic

[stffrdhrn]

One thought is when we actually write the spec changes is to not spec all of the numbers. Maybe exit, report and putc are enough for a first draft.

[stffrdhrn]

Some more links this is similar to:

• MIPS UHI (Unified Hosting Interface) - https://lists.gnu.org/archive/html/qemu-devel/2015-02/msg05390.html (no readily available spec anylonger)
• ARM Semihosting - linked in pr
• nios2 - Qemu semihosting
• m68k - Qemu semihosting
• xtensa Qemu semihosting

However, while the openrisc definition centers around very primative putc and report for writing to stdout. The other implementations provide standard input/output operations via open, close, write, read and lseek operations.

Perhaps we should follow other semihosting solutions.