1 files changed, 46 insertions, 0 deletions
diff --git a/newasm/parsers/register.s b/newasm/parsers/register.s
new file mode 100644
index 0000000..48ebf7a
--- /dev/null
+++ b/newasm/parsers/register.s
@@ -0,0 +1,46 @@
+.syntax unified
+.cpu cortex-m0plus
+.thumb
+
+.type register, %function
+.global register
+
+// 1 unexpected char
+// 2 invalid general-purpose register number
+
+// R4 input buffer
+// R2 output buffer
+
+register: PUSH    {LR}
+          LDRB    R0, [R4]
+r_test:   CMP     R0, 'R
+          BNE     special
+          ADDS    R4, 1       // consume R
+          BL      decimal     // consume a decimal number (result in R2)
+          BEQ     validate
+          POP     {PC}        // error code already in R0
+validate: CMP     R2, #12     // general purpose registers 0-12
+          BLS     success
+          MOVS    R0, #2      // invalid register number error code
+          POP     {PC}
+special:  LDRH    R0, [R4]      // get two bytes from input stream
+          ADR     R2, table     // get address of table
+          MOVS    R3, 0         // set table offset to 0
+loop:     LDRH    R1, [R2, R3]  // get two bytes from table at current offset
+          CMP     R0, R1        // compare input to table row
+          BEQ     done          // if equal then done
+          ADDS    R3, 2         // increment table offset
+          CMP     R3, 6         // compare offset to table size
+          BLO     loop          // loop until end of table
+          MOVS    R0, #1        // return code 1 (unexpected char)
+          POP     {PC}
+done:     ADDS    R4, 2         // consume two chars
+          LSRS    R3, 1         // divide table offset by two to row
+          ADDS    R3, 13        // add 13 to get register number
+          MOVS    R2, R3
+success:  MOVS    R0, #0        // return code 0 (success)
+          POP     {PC}
+
+table:    .ascii  "SP"
+          .ascii  "LR"
+          .ascii  "PC"