Reworked this all slightly:

* There was a pre-existing (undocumented) variable PICO_NO_STORED_VECTOR_TABLE that did something similar (have replaced that with the new var)
* Renamed PICO_MINIMAL_VECTOR_TABLE to PICO_MINIMAL_STORED_VECTOR_TABLE to contrast PICO_NO_RAM_VECTOR_TABLE
* Explicitly mentioned that PICO_MINIMAL_STORED_VECTOR_TABLE has no effect on RISC-V atm
* Added a new documented variable PICO_NUM_VTABLE_IRQs to indicate how many IRQs the user would like to store in the vtable (either stored or RAM) - this is the limit of what any IRQ api can touch, or indeed what isr_irqN you can override. This is a better alternative to changing PICO_RAM_VECTOR_TABLE_SIZE (which wasn't documented), but did allow you (unsafely) to limit the RAM vector table size (but not the stored size)

TLDR:

1. if you set PICO_MINIMAL_STORED_VECTOR_TABLE=1, you get a 4 word VTABLE on Arm.. you still get a RAM vector table with up to PICO_NUM_VTABLE_IRQs (defaults to NUM_IRQs) but everything - including things like `isr_pendsv` point to `__unhandled_user_irq`.
2. PICO_NUM_VTABLE_IRQs (default NUM_IRQS) sets the number of IRQs that have handlers in the VTABLE (this affects the space reserved, and what the irq APIs let you touch - at least as far as invalid_params_if!)
3. If you set PICO_MINIMAL_STORED_VECTOR_TABLE=1 for a no flash binary, then PICO_NUM_VTABLE_IRQs is forced to 0 as there is no reserved space for the handlers.

Author: kilograham

src/host/hardware_irq/include/hardware/irq.h

@@ -128,7 +128,7 @@ extern "C" {
 typedef void (*irq_handler_t)(void);
 
 static inline void check_irq_param(__unused uint num) {
-    invalid_params_if(HARDWARE_IRQ, num >= NUM_IRQS);
+    invalid_params_if(HARDWARE_IRQ, num >= PICO_NUM_VTABLE_IRQS);
 }
 
 /*! \brief Set specified interrupt's priority

src/host/hardware_irq/irq.c

@@ -96,7 +96,7 @@ void PICO_WEAK_FUNCTION_IMPL_NAME(irq_init_priorities)() {
 }
 
 static uint get_user_irq_claim_index(uint irq_num) {
-    invalid_params_if(HARDWARE_IRQ, irq_num < FIRST_USER_IRQ || irq_num >= NUM_IRQS);
+    invalid_params_if(HARDWARE_IRQ, irq_num < FIRST_USER_IRQ || irq_num >= PICO_NUM_VTABLE_IRQS);
     // we count backwards from the last, to match the existing hard coded uses of user IRQs in the SDK which were previously using 31
     static_assert(NUM_IRQS - FIRST_USER_IRQ <= 8, ""); // we only use a single byte's worth of claim bits today.
     return NUM_IRQS - irq_num  - 1u;

src/rp2040/pico_platform/include/pico/platform.h

@@ -66,10 +66,6 @@
 #define PICO_RP2040_B2_SUPPORTED 1
 #endif
 
-#ifndef PICO_RAM_VECTOR_TABLE_SIZE
-#define PICO_RAM_VECTOR_TABLE_SIZE (VTABLE_FIRST_IRQ + NUM_IRQS)
-#endif
-
 // PICO_CONFIG: PICO_CLKDIV_ROUND_NEAREST, True if floating point clock divisors should be rounded to the nearest possible clock divisor by default rather than rounding down, type=bool, default=1, group=pico_platform
 #ifndef PICO_CLKDIV_ROUND_NEAREST
 #define PICO_CLKDIV_ROUND_NEAREST 1

src/rp2350/pico_platform/include/pico/platform.h

@@ -54,10 +54,6 @@
 #define PICO_NO_RAM_VECTOR_TABLE 0
 #endif
 
-#ifndef PICO_RAM_VECTOR_TABLE_SIZE
-#define PICO_RAM_VECTOR_TABLE_SIZE (VTABLE_FIRST_IRQ + NUM_IRQS)
-#endif
-
 // PICO_CONFIG: PICO_USE_STACK_GUARDS, Enable/disable stack guards, type=bool, default=0, advanced=true, group=pico_platform
 #ifndef PICO_USE_STACK_GUARDS
 #define PICO_USE_STACK_GUARDS 0

src/rp2_common/hardware_irq/include/hardware/irq.h

@@ -23,6 +23,25 @@
 #define PICO_VTABLE_PER_CORE 0
 #endif
 
+// note this is defined here as it simplifies dependencies
+// PICO_CONFIG: PICO_MINIMAL_STORED_VECTOR_TABLE, Only store a very minimal vector table in the binary on Arm, type=bool, default=0, advanced=true, group=pico_crt0
+#ifndef PICO_MINIMAL_STORED_VECTOR_TABLE
+#define PICO_MINIMAL_STORED_VECTOR_TABLE 0
+#endif
+
+#if PICO_MINIMAL_STORED_VECTOR_TABLE && (PICO_NO_FLASH && !defined(__riscv))
+#if PICO_NUM_VTABLE_IRQS
+#warning PICO_NUM_VTABLE_IRQS is specied with PICO_MINIMAL_STORED_VECTOR_TABLE for NO_FLASH Arm binary; ignored
+#undef PICO_NUM_VTABLE_IRQS
+#endif
+#define PICO_NUM_VTABLE_IRQS 0
+#else
+// PICO_CONFIG: PICO_NUM_VTABLE_IRQS, Number of IRQ handlers in the vector table - can be lowered to save space if you aren't using some higher IRQs, type=int, default=NUM_IRQS, group=hardware_irq
+#ifndef PICO_NUM_VTABLE_IRQS
+#define PICO_NUM_VTABLE_IRQS NUM_IRQS
+#endif
+#endif
+
 #ifndef __ASSEMBLER__
 
 #include "pico.h"
@@ -195,7 +214,7 @@ extern "C" {
 typedef void (*irq_handler_t)(void);
 
 static inline void check_irq_param(__unused uint num) {
-    invalid_params_if(HARDWARE_IRQ, num >= NUM_IRQS);
+    invalid_params_if(HARDWARE_IRQ, num >= PICO_NUM_VTABLE_IRQS);
 }
 
 /*! \brief Set specified interrupt's priority

src/rp2_common/hardware_irq/irq.c

@@ -76,7 +76,7 @@ bool irq_is_enabled(uint num) {
 }
 
 static inline void irq_set_mask_n_enabled_internal(uint n, uint32_t mask, bool enabled) {
-    invalid_params_if(HARDWARE_IRQ, n * 32u >= ((NUM_IRQS + 31u) & ~31u));
+    invalid_params_if(HARDWARE_IRQ, n * 32u >= ((PICO_NUM_VTABLE_IRQS + 31u) & ~31u));
 #if defined(__riscv)
     if (enabled) {
         hazard3_irqarray_clear(RVCSR_MEIFA_OFFSET, 2 * n, mask & 0xffffu);
@@ -658,15 +658,15 @@ __weak void runtime_init_per_core_irq_priorities(void) {
         *p++ = prio4;
     }
 #else
-    for (uint i = 0; i < NUM_IRQS; ++i) {
+    for (uint i = 0; i < PICO_NUM_VTABLE_IRQS; ++i) {
         irq_set_priority(i, PICO_DEFAULT_IRQ_PRIORITY);
     }
 #endif
 #endif
 }
 
 static uint get_user_irq_claim_index(uint irq_num) {
-    invalid_params_if(HARDWARE_IRQ, irq_num < FIRST_USER_IRQ || irq_num >= NUM_IRQS);
+    invalid_params_if(HARDWARE_IRQ, irq_num < FIRST_USER_IRQ || irq_num >= PICO_NUM_VTABLE_IRQS);
     // we count backwards from the last, to match the existing hard coded uses of user IRQs in the SDK which were previously using 31
     static_assert(NUM_IRQS - FIRST_USER_IRQ <= 8, ""); // we only use a single byte's worth of claim bits today.
     return NUM_IRQS - irq_num  - 1u;

src/rp2_common/pico_crt0/crt0.S

@@ -11,6 +11,7 @@
 
 #include "hardware/regs/addressmap.h"
 #include "hardware/regs/sio.h"
+#include "hardware/irq.h"
 #include "pico/binary_info/defs.h"
 #include "boot/picobin.h"
 #include "pico/bootrom.h"
@@ -23,26 +24,22 @@
 
 pico_default_asm_setup
 
+#ifdef PICO_NO_STORED_VECTOR_TABLE
+#warning PICO_NO_STORED_VECTOR_TABLE is no longer used. PICO_MINIMAL_STORED_VECTOR_TABLE is not identical but usually serves the same purpose
+#endif
+
 .section .vectors, "ax"
 .align 2
 
-// PICO_CONFIG: PICO_MINIMAL_VECTOR_TABLE, Use Minimal Vector Table - this prevents use of all interrupts, type=bool, default=0, advanced=true, group=pico_crt0
-#ifndef PICO_MINIMAL_VECTOR_TABLE
-#define PICO_MINIMAL_VECTOR_TABLE 0
-#endif
-
-.global __vectors, __VECTOR_TABLE
+.global __vectors, __VECTOR_TABLE, __vectors_end
 __VECTOR_TABLE:
 __vectors:
 .word __StackTop
 .word _reset_handler
 
-#if PICO_MINIMAL_VECTOR_TABLE
 .word isr_invalid   // NMI
 .word isr_invalid   // HardFault
-#else
-.word isr_nmi
-.word isr_hardfault
+#if !PICO_MINIMAL_STORED_VECTOR_TABLE
 .word isr_invalid // Reserved, should never fire
 .word isr_invalid // Reserved, should never fire
 .word isr_invalid // Reserved, should never fire
@@ -55,15 +52,9 @@ __vectors:
 .word isr_invalid // Reserved, should never fire
 .word isr_pendsv
 .word isr_systick
-#if PICO_NO_STORED_VECTOR_TABLE && !PICO_NO_FLASH // note in no flash binary, we only have the single RAM vector table anyway
-#if PICO_NO_RAM_VECTOR_TABLE
-#error Can't specify PICO_NO_STORED_VECTOR_TABLE and PICO_NO_RAM_VECTOR_TABLE
-#endif
-// we don't include any IRQ vectors; we will initialize them during runtime_init in the RAM vector table
-#else
 
 .macro if_irq_word num func
-.if \num < NUM_IRQS
+.if \num < PICO_NUM_VTABLE_IRQS
 .word \func
 .endif
 .endm
@@ -152,11 +143,11 @@ if_irq_word 76 isr_irq76
 if_irq_word 77 isr_irq77
 if_irq_word 78 isr_irq78
 if_irq_word 79 isr_irq79
-#if NUM_IRQS > 80
+#if PICO_NUM_VTABLE_IRQS > 80
 #error more IRQ entries required
 #endif
-#endif
-#endif  // #if !PICO_MINIMAL_VECTOR_TABLE
+#endif  // #if !PICO_MINIMAL_STORED_VECTOR_TABLE
+__vectors_end:
 
 // all default exception handlers do nothing, and we can check for them being set to our
 // default values by seeing if they point to somewhere between __defaults_isrs_start and __default_isrs_end
@@ -177,7 +168,7 @@ __default_isrs_start:
 
 // these are separated out for clarity
 decl_isr_bkpt isr_invalid
-#if !PICO_MINIMAL_VECTOR_TABLE
+#if !PICO_MINIMAL_STORED_VECTOR_TABLE
 decl_isr_bkpt isr_nmi
 decl_isr_bkpt isr_hardfault
 decl_isr_bkpt isr_svcall
@@ -189,7 +180,7 @@ __default_isrs_end:
 
 .altmacro
 .macro decl_isr name
-#if !PICO_NO_STORED_VECTOR_TABLE | PICO_NO_FLASH
+#if !PICO_MINIMAL_STORED_VECTOR_TABLE | PICO_NO_FLASH
 // We declare a weak label, so user can override
 .weak \name
 #else
@@ -201,8 +192,13 @@ __default_isrs_end:
 .endm
 
 .macro if_irq_decl num func
-.if \num < NUM_IRQS
+.if \num < PICO_NUM_VTABLE_IRQS
 decl_isr \func
+.elseif \num < NUM_IRQS
+// We declare a strong label, so user can't override, since there is no vtable entry
+.type \func,%function
+.thumb_func
+\func:
 .endif
 .endm
 
@@ -286,10 +282,11 @@ if_irq_decl 76 isr_irq76
 if_irq_decl 77 isr_irq77
 if_irq_decl 78 isr_irq78
 if_irq_decl 79 isr_irq79
-#if NUM_IRQS > 80
+#if PICO_NUM_VTABLE_IRQS > 80
 #error more IRQ entries required
 #endif
 
+#endif  // #if !PICO_MINIMAL_STORED_VECTOR_TABLE
 // All unhandled USER IRQs fall through to here
 .global __unhandled_user_irq
 .thumb_func
@@ -299,7 +296,6 @@ __unhandled_user_irq:
 .global unhandled_user_irq_num_in_r0
 unhandled_user_irq_num_in_r0:
     bkpt #0
-#endif  // #if !PICO_MINIMAL_VECTOR_TABLE
 
 // ----------------------------------------------------------------------------
 

src/rp2_common/pico_crt0/crt0_riscv.S

@@ -8,9 +8,10 @@
 
 #include "hardware/regs/addressmap.h"
 #include "hardware/regs/rvcsr.h"
+#include "hardware/irq.h"
 #include "pico/binary_info/defs.h"
-#include "boot/picobin.h"
 #include "pico/bootrom_constants.h"
+#include "boot/picobin.h"
 
 #ifdef NDEBUG
 #ifndef COLLAPSE_IRQS
@@ -160,64 +161,22 @@ check_irq_before_exit:
     addi sp, sp, 80
     mret
 
+
 // Default software vector table for system interrupts, routed through
 // mip.meip. Note this is assumed in e.g. hardware_irq to begin exactly 0x34
 // words after the hardware vector table indicated by mtvec (defined above).
 .p2align 4
 .global __soft_vector_table
 __soft_vector_table:
-.word isr_irq0
-.word isr_irq1
-.word isr_irq2
-.word isr_irq3
-.word isr_irq4
-.word isr_irq5
-.word isr_irq6
-.word isr_irq7
-.word isr_irq8
-.word isr_irq9
-.word isr_irq10
-.word isr_irq11
-.word isr_irq12
-.word isr_irq13
-.word isr_irq14
-.word isr_irq15
-.word isr_irq16
-.word isr_irq17
-.word isr_irq18
-.word isr_irq19
-.word isr_irq20
-.word isr_irq21
-.word isr_irq22
-.word isr_irq23
-.word isr_irq24
-.word isr_irq25
-.word isr_irq26
-.word isr_irq27
-.word isr_irq28
-.word isr_irq29
-.word isr_irq30
-.word isr_irq31
-.word isr_irq32
-.word isr_irq33
-.word isr_irq34
-.word isr_irq35
-.word isr_irq36
-.word isr_irq37
-.word isr_irq38
-.word isr_irq39
-.word isr_irq40
-.word isr_irq41
-.word isr_irq42
-.word isr_irq43
-.word isr_irq44
-.word isr_irq45
-.word isr_irq46
-.word isr_irq47
-.word isr_irq48
-.word isr_irq49
-.word isr_irq50
-.word isr_irq51
+.macro vtable_irq_n n
+.word isr_irq\n
+.endm
+
+.set IRQN, 0
+.rept PICO_NUM_VTABLE_IRQS
+vtable_irq_n IRQN
+.set IRQN, IRQN + 1
+.endr
 
 // all default trap handlers do nothing, and we can check for them being set to our
 // default values by seeing if they point to somewhere between __defaults_isrs_start and __default_isrs_end
@@ -252,58 +211,23 @@ decl_isr isr_riscv_machine_exception
 decl_isr_bkpt isr_riscv_machine_timer
 decl_isr_bkpt isr_riscv_machine_soft_irq
 
-decl_isr isr_irq0
-decl_isr isr_irq1
-decl_isr isr_irq2
-decl_isr isr_irq3
-decl_isr isr_irq4
-decl_isr isr_irq5
-decl_isr isr_irq6
-decl_isr isr_irq7
-decl_isr isr_irq8
-decl_isr isr_irq9
-decl_isr isr_irq10
-decl_isr isr_irq11
-decl_isr isr_irq12
-decl_isr isr_irq13
-decl_isr isr_irq14
-decl_isr isr_irq15
-decl_isr isr_irq16
-decl_isr isr_irq17
-decl_isr isr_irq18
-decl_isr isr_irq19
-decl_isr isr_irq20
-decl_isr isr_irq21
-decl_isr isr_irq22
-decl_isr isr_irq23
-decl_isr isr_irq24
-decl_isr isr_irq25
-decl_isr isr_irq26
-decl_isr isr_irq27
-decl_isr isr_irq28
-decl_isr isr_irq29
-decl_isr isr_irq30
-decl_isr isr_irq31
-decl_isr isr_irq32
-decl_isr isr_irq33
-decl_isr isr_irq34
-decl_isr isr_irq35
-decl_isr isr_irq36
-decl_isr isr_irq37
-decl_isr isr_irq38
-decl_isr isr_irq39
-decl_isr isr_irq40
-decl_isr isr_irq41
-decl_isr isr_irq42
-decl_isr isr_irq43
-decl_isr isr_irq44
-decl_isr isr_irq45
-decl_isr isr_irq46
-decl_isr isr_irq47
-decl_isr isr_irq48
-decl_isr isr_irq49
-decl_isr isr_irq50
-decl_isr isr_irq51
+// Declare all the ISR labels
+.macro decl_isr_n n
+.if \n < PICO_NUM_VTABLE_IRQS
+    decl_isr isr_irq\n
+.elseif \n < NUM_IRQS
+    // We declare a strong label, so user can't override, since there is no vtable entry
+    .type isr_irq\n,%function
+    .thumb_func
+    isr_irq\n:
+.endif
+.endm
+
+.set IRQN, 0
+.rept PICO_NUM_VTABLE_IRQS
+decl_isr_n IRQN
+.set IRQN, IRQN + 1
+.endr
     // fall through
 
 // All unhandled USER IRQs fall through to here. Note there is no way to get