Move CSR space to 0x8000_0000

Author: mcejp

MEMORY_MAP.md

@@ -21,14 +21,13 @@ Note: might want to take advantage of RV32's single-instruction calls relative t
 
     bit  31     -> cache bypass
     bits 26..24 -> region (3 bits)
-                    0 = trap
-                    1 = I/O space
+                    0 = control/status registers   (TODO: trap if accessed with bit 31 cleared)
                     3 = rom
                     4..7 = sdram    (in other words: bit 26 -> SDRAM(1) / non-SDRAM(0))
     bit  23..0  -> address within 16M region
                     (not necessarily fully decoded)
 
-CPU external address bus is effectively 28 bits wide.
+CPU external address bus is effectively 27 bits wide (though bit 31 is also used to trap NULL pointer dereferences).
 
 
 ## Detailed description

firmware/prebuilt/test_dhrystone.bin

@@ -884,10 +884,10 @@ Disassembly of section .text:
 040009b0 <_write>:
  40009b0:	00060513          	mv	a0,a2
  40009b4:	00000793          	li	a5,0
- 40009b8:	810006b7          	lui	a3,0x81000
+ 40009b8:	800006b7          	lui	a3,0x80000
  40009bc:	00a7c463          	blt	a5,a0,40009c4 <_write+0x14>
  40009c0:	00008067          	ret
- 40009c4:	0106a703          	lw	a4,16(a3) # 81000010 <__stack+0x7b000010>
+ 40009c4:	0106a703          	lw	a4,16(a3) # 80000010 <__stack+0x7a000010>
  40009c8:	00177713          	andi	a4,a4,1
  40009cc:	fe071ce3          	bnez	a4,40009c4 <_write+0x14>
  40009d0:	00f58733          	add	a4,a1,a5

firmware/prebuilt/test_dhrystone.objdump.txt

@@ -1,7 +1,7 @@
 `ifndef MEMORY_MAP_SV
 `define MEMORY_MAP_SV
 
-localparam ADDR_CSR_START   = 27'h1000000;
+localparam ADDR_CSR_START   = 27'h0000000;
 localparam ADDR_SDRAM_START = 27'h4000000;
 
 function automatic addr_is_csr(input[26:0] addr);

rtl/memory_map.sv

@@ -11,7 +11,7 @@ enum { SDRAM_START          = 0x04000000 };
 
 /* Control/status registers (with cache bypass bit set) */
 
-#define _HW (*(struct top volatile*)0x81000000)
+#define _HW (*(struct top volatile*)0x80000000)
 
 
 /* RISC-V stuff */