diff --git a/src/assembler/inst_args.h b/src/assembler/inst_args.h
index 8e43585..b900f83 100644
--- a/src/assembler/inst_args.h
+++ b/src/assembler/inst_args.h
@@ -34,7 +34,8 @@ typedef enum {
 
 typedef struct {
     ASMArgImmType mask;
-    uint16_t value;
+    uint16_t uval;
+    int16_t sval;
 } ASMArgImmediate;
 
 typedef struct {
diff --git a/src/assembler/instructions.c b/src/assembler/instructions.c
index f6d8071..dbe97e6 100644
--- a/src/assembler/instructions.c
+++ b/src/assembler/instructions.c
@@ -147,35 +147,30 @@ static uint8_t fill_bytes_variadic(uint8_t *bytes, size_t len, ...)
 static ASMErrorDesc parse_arg(
     ASMInstArg *arg, const char *str, size_t size, char **symbol)
 {
-    // TODO
+#define USE_PARSER(func, argtype, field)                                      \
+    if (argparse_##func(&arg->data.field, str, size)) {                       \
+        arg->type = argtype;                                                  \
+        return ED_NONE;                                                       \
+    }
 
-    // AT_REGISTER
-    // AT_IMMEDIATE
-    // AT_INDIRECT
-    // AT_INDEXED
-    // AT_LABEL
-    // AT_CONDITION
+    DEBUG("parse_arg(): -->%.*s<-- %zu", (int) size, str, size)
+    USE_PARSER(register, AT_REGISTER, reg)
+    USE_PARSER(immediate, AT_IMMEDIATE, imm)
 
-    // ASMArgRegister reg;
-    // ASMArgImmediate imm;
+    // AT_INDIRECT
     // ASMArgIndirect indirect;
-    // ASMArgIndexed index;
-    // ASMArgLabel label;
-    // ASMArgCondition cond;
 
-    DEBUG("parse_arg(): -->%.*s<-- %zu", (int) size, str, size)
+    // AT_INDEXED
+    // ASMArgIndexed index;
 
-    if (parse_register(&arg->data.reg, str, size)) {
-        arg->type = AT_REGISTER;
-        return ED_NONE;
-    }
+    USE_PARSER(condition, AT_CONDITION, cond)
 
-    if (parse_condition(&arg->data.cond, str, size)) {
-        arg->type = AT_CONDITION;
-        return ED_NONE;
-    }
+    // AT_LABEL
+    // ASMArgLabel label;
 
     return ED_PS_ARG_SYNTAX;
+
+#undef USE_PARSER
 }
 
 /*
@@ -193,6 +188,8 @@ static ASMErrorDesc parse_args(
     while (i < size) {
         char c = str[i];
         if (c == ',') {
+            if (i == start)
+                return ED_PS_ARG_SYNTAX;
             if ((err = parse_arg(&args[*nargs], str + start, i - start, &symbol)))
                 return err;
             (*nargs)++;
@@ -201,7 +198,9 @@ static ASMErrorDesc parse_args(
             if (i < size && str[i] == ' ')
                 i++;
             start = i;
-            if (*nargs >= 3 && i < size)
+            if (i == size)
+                return ED_PS_ARG_SYNTAX;
+            if (*nargs >= 3)
                 return ED_PS_TOO_MANY_ARGS;
         } else {
             if ((c >= 'a' && c <= 'z') || (c >= '0' && c <= '9') ||
diff --git a/src/assembler/parse_util.c b/src/assembler/parse_util.c
index 6dbd586..e6362c1 100644
--- a/src/assembler/parse_util.c
+++ b/src/assembler/parse_util.c
@@ -177,7 +177,7 @@ bool parse_bytes(uint8_t **result, size_t *length, const char *arg, ssize_t size
 /*
     Read in a register argument and store it in *result.
 */
-bool parse_register(ASMArgRegister *result, const char *arg, ssize_t size)
+bool argparse_register(ASMArgRegister *result, const char *arg, ssize_t size)
 {
     if (size < 1 || size > 3)
         return false;
@@ -232,7 +232,7 @@ bool parse_register(ASMArgRegister *result, const char *arg, ssize_t size)
 /*
     Read in a register argument and store it in *result.
 */
-bool parse_condition(ASMArgCondition *result, const char *arg, ssize_t size)
+bool argparse_condition(ASMArgCondition *result, const char *arg, ssize_t size)
 {
     if (size < 1 || size > 2)
         return false;
@@ -265,6 +265,55 @@ bool parse_condition(ASMArgCondition *result, const char *arg, ssize_t size)
 }
 
 /*
+    Read in an immediate argument and store it in *result.
+*/
+bool argparse_immediate(ASMArgImmediate *result, const char *arg, ssize_t size)
+{
+    bool negative = false;
+    ssize_t i = 0;
+
+    if (size <= 0)
+        return false;
+
+    while (arg[i] == '-' || arg[i] == '+' || arg[i] == ' ') {
+        if (arg[i] == '-')
+            negative = !negative;
+        if (++i >= size)
+            return false;
+    }
+
+    uint32_t uval;
+    if (parse_uint32_t(&uval, arg, size) && uval > UINT16_MAX)
+        return false;
+
+    int32_t sval = negative ? uval : -uval;
+    if (sval < INT16_MIN)
+        return false;
+
+    result->uval = uval;
+    result->sval = sval;
+    result->mask = 0;
+
+    if (negative) {
+        if (sval >= INT8_MIN && sval <= INT8_MAX)
+            result->mask |= IMM_S8;
+        if (sval >= INT8_MIN + 2 && sval <= INT8_MAX + 2)
+            result->mask |= IMM_REL;
+    } else {
+        result->mask = IMM_U16;
+        if (uval <= UINT8_MAX)
+            result->mask |= IMM_U8;
+        if (uval <= 7)
+            result->mask |= IMM_BIT;
+        if (!(uval & ~0x38))
+            result->mask |= IMM_RST;
+        if (uval <= 2)
+            result->mask |= IMM_IM;
+    }
+    return true;
+}
+
+/*
     Read in a boolean argument from the given line and store it in *result.
 */
 DIRECTIVE_PARSE_FUNC(bool, bool)
diff --git a/src/assembler/parse_util.h b/src/assembler/parse_util.h
index 44b98bf..38c06be 100644
--- a/src/assembler/parse_util.h
+++ b/src/assembler/parse_util.h
@@ -13,19 +13,22 @@
 
 /* Functions */
 
+/* General parsers */
 bool parse_bool(bool*, const char*, ssize_t);
 bool parse_uint32_t(uint32_t*, const char*, ssize_t);
 bool parse_string(char**, size_t*, const char*, ssize_t);
 bool parse_bytes(uint8_t**, size_t*, const char*, ssize_t);
 
-bool parse_register(ASMArgRegister*, const char*, ssize_t);
-bool parse_condition(ASMArgCondition*, const char*, ssize_t);
+/* Instruction argument parsers */
+bool argparse_register(ASMArgRegister*, const char*, ssize_t);
+bool argparse_condition(ASMArgCondition*, const char*, ssize_t);
+bool argparse_immediate(ASMArgImmediate*, const char*, ssize_t);
 
+/* Preprocessor directive parsers */
 bool dparse_bool(bool*, const ASMLine*, const char*);
 bool dparse_uint32_t(uint32_t*, const ASMLine*, const char*);
 bool dparse_uint16_t(uint16_t*, const ASMLine*, const char*);
 bool dparse_uint8_t(uint8_t*, const ASMLine*, const char*);
-
 bool dparse_rom_size(uint32_t*, const ASMLine*, const char*);
 bool dparse_region_string(uint8_t*, const ASMLine*, const char*);
 bool dparse_size_code(uint8_t*, const ASMLine*, const char*);
diff --git a/src/assembler/tokenizer.c b/src/assembler/tokenizer.c
index 93baade..9e51f3e 100644
--- a/src/assembler/tokenizer.c
+++ b/src/assembler/tokenizer.c
@@ -63,11 +63,11 @@ static ErrorInfo* add_label_to_table(
     ASMSymbolTable *symtable, const ASMLine *line, size_t offset, int8_t slot)
 {
     ASMArgRegister reg;
-    if (parse_register(&reg, line->data, line->length - 1))
+    if (argparse_register(&reg, line->data, line->length - 1))
         return error_info_create(line, ET_SYMBOL, ED_SYM_IS_REGISTER);
 
     ASMArgCondition cond;
-    if (parse_condition(&cond, line->data, line->length - 1))
+    if (argparse_condition(&cond, line->data, line->length - 1))
         return error_info_create(line, ET_SYMBOL, ED_SYM_IS_CONDITION);
 
     char *symbol = strndup(line->data, line->length - 1);